Five anomalous structural aspects of rift basins in Thailand and their impact on petroleum systems

ByChris K. Morley

PTTEP, 27th Floor, ENCO Building, Soi 11, Vibhavadi-Rangsit Road, Chatuchak, Bangkok, Thailand, 10900chrissmorley@gmail.com

AbstractThe intra-cratonic, supra-subduction zone setting of the Thailand rift basins caused the rifts to evolve in several ways differently from other intra-cratonic rifts. Key differences include: widespread occurrence of low-angle normal faults; basin inversion alternating with rifting; diachronous initiation and cessation of rifting; rapid post-rift subsidence and extensive, low-displacement post-rift faults. These characteristics are related to hot, weak continental lithosphere, rapid evolution of the plate boundaries and stresses during the Cenozoic, and the history of subduction and accretion. Low-angle normal faults impacted the Sirikit Field by controlling the location of fluvio-deltaic reservoirs interfingering lacustrine shales (source and seal). Basin

inversion alternating with syn-rift section is detrimental to prospectivity of the southern half of the Phitsa-nulok Basin by destroying syn-rift structure, and halting hydrocarbon maturation early in the basin history. The diachronous timing of basin development resulted in large, gas-dominated hydrocarbon accumulations in the eastern Gulf of Thailand basins, but was detrimental for younger basins that lack the post-rift section, and the greater variety of petroleum systems and traps that post-rift basins offer. The extensive low displace-ment-length ratio post-rift faults form significant hydrocarbon traps in the Pattani and North Malay basins.

They represent a structural style not usually found in post-rift basins.

IntroductionThe Cenozoic rift system that runs through central Thailand is about 1,900 km long, and up to 300

km wide. The northern part of the rift system lies 800 km south of the Eastern Himalayan Syntaxis, while the westernmost part of the rift system lies 900 km east of the Andaman trench (Fig. 1). West of the rift

basins a back-arc rift system is present in the Andaman Sea area, this rift system is located at most 600 km from the trench (Figs. 1 and 2). The continental core of Southeast Asia is called Sundaland (e.g. Hall and

Morley, 2004; Fig. 1). Thailand rift basins form a failed rift system in the central part of Sundaland. Al-though the basins are remotely associated with the Himalayan collisional setting, and the Andaman-Sumatra subduction zone, they lie too far from these areas to be called either collisional or back-arc basins, but their

development is clearly influenced by these settings (e.g. Hall and Morley, 2004).

As outlined above the rift basins of Thailand have developed in a tectonic setting very different from typical continental rifts in old cratonic areas such as the East African Rift System, Central African Rift System, and the North Sea. Yet all these basins can be classed as failed rifts, or intracratonic rifts. As a way

of highlighting the differences within this class of rift, this paper investigates five structural aspects of the

rift basins in Thailand that are anomalous with respect to typical failed rifts such as the ones listed above, describes how these aspects have influenced hydrocarbon prospectivity, and finally discusses the tectonic

characteristics of SE Asia that have influenced these anomalous features.

The five aspects considered in this paper are: 1) highly diachronous timing of rift basin cessation

along the system, 2) Low-angle normal faults, 3) inversion alternating with extension, 4) very rapid post-rift

subsidence, and 5) extensive extensional faulting in the post-rift section.

Geological background Both rifting and strike-slip deformation have extensively affected onshore and offshore Thailand dur-

ing the Cenozoic (Polachan et al., 1991; Lacassin et al., 1997; Morley, 2004; Morley et al., 2011). The onset

of rift basin formation ranges in age from Late Eocene (e.g. Krabi Basin) to Late Miocene, (e.g. Phitsanulok

Basin), (Morley and Racey, 2011). The rift basins are at their broadest in the Gulf of Thailand, and extend

upwards through central Thailand to the northernmost part of the country (Fig. 3). The rift basins of the

Gulf of Thailand to northern Thailand are dominantly or entirely filled by continental deposits (Fig. 4). It is

only in the post-rift phase that marine incursions become a feature of the Gulf of Thailand basins (Morley

and Racey, 2011). In contrast the Mergui and Andaman syn-rift basins of the Andaman Sea region (Fig. 3)

are dominantly filled by marine and fluvio-deltatic deposits. The Mergui and Andaman Basins are not dis-cussed in detail in this paper because they contain no commercial hydrocarbon discoveries at present.

The nature of the interactions between strike-slip faulting and rifting in Thailand is controversial. Clearly there are examples where strike-slip faults are linked to rift basins, in some cases these relationships can be interpreted as transfer faults (e.g. Fang Basin, Morley, 2007), but there are also clear examples where

the basins lie at releasing bends associated with strike-slip faults (e.g. Mae Sot Basin, Mae Ping Fault zone,

and Pracham, Sisawat and Huai Malai basins along the Three Pagodas Faults, Morley and Racey, 2011,

Morley et al., 2011). Some workers have sought to explain all the rift basins in a strike-slip context (e.g.

Tapponnier et al., 1986; Polachan et al., 1991). However, it is the opinion of the author that the evolution of

the basins is more complex, and that most of the Cenozoic basins opened under an extensional stress regime with the strike-slip faults either activating at different times from extension, and/or that there is a spatial

transition from areas of strike-slip to extensional dominated stress regimes (e.g. Morley, 2002b, 2007; Mor-ley et al., 2011).

The most prolific hydrocarbon province lies in the eastern Gulf of Thailand, where the Pattani Basin

and North Malay basin produce predominantly gas with some liquids, primarily from structural and struc-tural-stratigraphic traps associated with conjugate fault sets in the post-rift section (e.g. Racey, 2011). Total

produced gas and remaining proven plus probable reserves of about 30 TCF gas are estimated for the eastern Gulf of Thailand, together with about 360 MMbbl oil and 723 MMbbl condensate (Racey, 2011). A small

number of scattered oil fields are present in the western Gulf of Thailand. Oil production from the western

Gulf may in the near future attain a peak of about 100,000 bbl/day from the Kra (Manora Field, ~20-29

MMbbl, recoverable reserves) Western (Bualuang oil field, recoverable reserves ~20 MMbbl), Songkhla-Na-khon (recoverable reserves estimated between 10-28 MMbbl), and Chumphon basins. Onshore, small fields

have been found in the Fang, Suphan Buri and Phetchabun basins. Sirkit field is the largest oil field in all of

Thailand and lies within the Phitsanulok Basin (Flint et al, 1988; Morley et al., 2007a). Total output from

the field is about 200 MMbbl oil, with perhaps a further 20% yet to be produced.

The general structural style of the rifts comprises typical arrangements of half grabens found in rifts, which change polarity and give rise to a variety of transfer zones (Morley et al., 1990). Overall the rifts trend

N-S, but the influence of pre-existing fabrics causes NE-SW and NW-SE and even ENE-WSW trends to be

activated in places (Morley et al., 2004).

Diachronous termination of riftingRift initiation and termination prior to the development of oceanic crust is commonly related to

either an aulacogene setting (failed arm of a triple junction; Burke, 1977; Fig. 5), or the abandoned propa-gating tip of a rift system (Morley 2002a). The best examples of propagating rifts are along the successful

rifts that have gone on to form passive margins, for example the Jurassic to Cretaceous break-up along a 9000 km long segment of the African margin. Aulacogenes lie at a high angle to the passive margin rifts, and are related in timing to the evolution of the adjacent passive margin segment. In the simplest situations

these failed rifts tend to develop along their length over a short period of time and propagate together from a number of initial seed points or propagate inland with time (Genik, 1994; Koehn et al., 2008; Roberts et al.,

2012; Fig. 5), and terminate activity once seafloor spreading begins on the adjacent rift margin, unless an ac-tive component to the driving forces remains (e.g. East African Rift, Nyblade, 2002). However, once created

the rifts are zones of weakness that may be subsequently reactivated multiple times.

A good example of the basic pattern outlined above are the Jurassic syn-rift sub-basins of the North

Sea. These basins show a diachronous onset of Late Jurassic extension with rifting propagating towards the centre of a pre-rift dome with time (Rattey and Hayward, 1993). The termination of this short (< 15 my) pe-riod of extension across almost the entire rift system is marked by the base Cretaceous or late Cimmerian unconformity (e.g. Kyrkjebo et al., 2004). This termination is attributed to the onset of seafloor spreading in

the Atlantic (e.g. Rattey and Hayward, 1993). Periods of failed rift activity related to abandoned propagat-ing rift tips are represented by the Gulf of Suez (~15 Ma duration, Bosworth et al., 2005) and the Reconcavo

Basin (~18 Ma duration, Magnevita et al., 2005).

The Central African Rift is 3,800 km long, traverses the entire length of central Africa, and exhibits a remarkably uniform Early Cretaceous onset of rifting in most basins, and termination of initial rifting around the mid to late Albian (Genik, 1993; Keller et al., 2006). However, subsequent extensional reactivation of

the rifts continued into the Palaeogene, long after mid-Cretaceous seafloor spreading was initiated in the At-lantic (Genik, 1994; Keller et al., 2006). Fairhead et al. (2013) show how three distinct phases of extension

in the Central African Rift system (particularly the Muglad Basin, Sudan) can be correlated with azimuthal

changes in Atlantic ocean fracture zones. The Sirte Basin is another example of pulsed basin reactivation (Abdulbaset et al., 2008).

The East African Rift system exhibits complex timing of basins in part related to propagation of the rift system to the south to southwest with time, and interactions with mantle plumes (Nyblade, 2002). The

Eastern Branch in the Turkana area shows Eocene-Oligocene volcanism followed by Oligocene basin devel-opment (Morley et al., 1999). Oligocene basin development is consistent with the onset of extension in the

Gulf of Aden (Bosworth et al., 2005). Deformation has propagated to the south in Kenya and Tanzania with

time (Morley, 1999; Le Gall et al., 2004). Although Oligocene syn-rift sediments have been identified for

the Lake Rukwa segment in the Western Branch (Roberts et al., 2012), much of the 2000 km long Western

Branch development is Neogene in age (Ebinger, 1989). There are also newly developing rift segments of

Holocene age to the SW of the Western Branch, such as the Okavango Rift Zone (Kinabo et al., 2008).

The rift basins of Thailand show considerable diachroneity in timing both regarding the onset and termination of rifting (in other failed rifts it tends to be one but not the other), in general showing a younging

of both rift onset and termination to the north with time (Fig. 4). There is also an east to west shift in activ-ity within the Gulf of Thailand. As reviewed in Morley and Racey (2011) in the Gulf of Thailand the oldest

rifts (Late Eocene-Oligocene) occur in the eastern and southern portion of the Gulf. Rifting ceased around

the Oligocene-Miocene boundary in the eastern Gulf, while in the western Gulf the story is more diverse.

Some basins in the southwest of the Gulf show both Late Eocene and Early Miocene phases of extension. While in the NW part of the Gulf the rift basins are predominantly of Late Oligocene-Early Miocene age.

Onshore in Central Thailand and in NW Thailand to the Chiang Mai and Fang basins, rifting lasted from

Late Oligocene to Late Miocene. In eastern northern Thailand some rift basins appear to only be of Middle

Miocene or Middle to Late Miocene age (Morley et al., 2001). The northwards younging of rifting is also

observed in the Mergui Basin (Srisuriyon and Morley, in review). The difference in age of the onset of rift-ing from south to north is moderate, perhaps about 10 my, while the difference in the age of rift termination is around 15-20 my. The result is that some basins have been accumulating sediment in thick post-rift basins,

while contemporaneously other basins have continued to accumulate sediments in active half grabens. In a review of hydrocarbon resources in different rift basin and post-rift basin associations Morley

(1999) concluded that simple rifts (i.e. failed rifts without a post-rift basin) tended to have small resources

compared with rifts capped either by a sag basin (failed rift) or passive margin (Fig. 6). The absence of the

capping post-rift basin meant that there were problems with sealing potential reservoir units in large traps, and that maturation of the source rock was likely to be limited to the deepest parts of the basin. The post-rift basin can also be the site of completely different plays, and adds to the potential diversity of the petroleum systems. The Reconcavo Basin, Brazil, is the most prolific example of hydrocarbons in a simple rift. From

the start of exploration in 1937 to December 2001 5,626 wells were drilled in the basin resulting in cumu-lative production of 1.4 billion bbl oil and 1.9 TCF gas. By comparison the North Sea comprises two rift

systems (Triassic and Late Jurassic) capped by post-rift basins. The largest field in the Cretaceous-Cenozoic

post rift section is Forties Field (~5 billion bbl oil in place), while one of the largest syn-rift fields, Gullfaks,

has produced more oil (~2.4 billion barrels) than the entire Reconcavo Basin.

The discoveries in Lake Albert in the past 7 years have added another example of a highly suc-cessful simple rift play, with about 1.2 billion barrels of recoverable oil (http://www.tullowoil.com/index.

asp?pageid=282). Contributing factors to the success of the Reconcavo Basin are the unusually high qual-ity of lacustrine turbidite sandstone reservoirs, and the presence of good pre-rift reservoir sands sealed by overlying lacustrine shales. Very high geothermal gradients are a significant factor in maturing the young

sediment in Lake Albert. The fundamental differences in hydrocarbon plays between simple rift basins and rifts with sag basins discussed above is clearly seen in the Thailand rift basins. The rift basins that were active during the Miocene have thin, poorly developed to virtually absent post-rift basin section, and they can essentially be classed as simple syn-rift basins. The basins of the eastern Gulf have a very well developed Neogene post-

rift section up to 6 km thick that is the primary hydrocarbon exploration target. As mentioned in the geologi-cal background section the eastern gulf (syn-rift plus post-rift) contains the bulk of the hydrocarbon reserves

of the country and is gas-prone, while the western gulf and onshore rift basins (simple rift) contains relative-

ly small oil fields, with Sirikit Field being the one notable exception (Racey, 2011).

While the generalization regarding the syn-rift vs post-rift-dominated plays works well, there are

anomalous distributions of fields within these two classes of basin that require explanation. For example, de-spite the general small field size (up to 10s mmbbo) in the simple rifts, there is the Sirkit Field in this class

of rift, which has produced over 200 mmbbo to date. The reason for the occurrence of the Sirikit field is

discussed in the low-angle normal faults section. In the eastern Gulf there are two main trends of basins, the

gas-rich Pattani-North Malay basin trend, and the Khmer Basin, which at present has only small, marginally

economic discoveries. Two anomalous features of the Pattani and North Malay basins are the great depth

and rapid subsidence exhibited by the post-rift basins, and the extensive swarms of low-displacement nor-mal faults that set up the trapping configurations (Rigo de Rhigi et al., 2002; Kornsawan and Morley, 2002).

These aspects are discussed in subsequent sections.

Low-angle normal faults Typical failed continental rifts are considered to be composed predominantly of normal faults initi-ated at a high-angle (e.g. Buck, 1991). This appears to be the case for the majority of the major boundary

faults in the East African Rift (e.g. Morley, 1989), the Gulf of Suez (Jackson et al., 1988) and the North Sea

(Fossen et al., 2000). Low-angle faults in these settings tend form either the detachment zones flattening out

deep within the crust, or they are the result of rotation of initially high-angle faults, a few may also result from reactivation of low-angle pre-existing orogenic structures (e.g. Fossen et al., 2000, Lyngsie, 2007). The

occurrence of high-extension, metamorphic core-complex-type low-angle normal faults is generally ex-plained either by atypical crustal conditions (such as the hot crust or over-thickened crust of orogenic belts),

together with rotation of initially high-angle faults (e.g. Wernicke and Axen, 1988; see review in Whitney et

al., 2013).

Six low-angle normal faults (LANFs) bound half-graben basins in the Gulf of Thailand-onshore rift

basin trend, two bound crystalline basement in the footwall block and are seen in outcrop adjacent to the Gulf of Thailand (Rayong, and Khanom), while four occur in the Mergui Basin. Morley (2009) described the

occurrence of LANFs in Thailand and noted that all of them dip eastwards between about 20 and 30. What

differentiates these basins from typical failed rifts is: a) the presence in some areas of metamorphic core

complex associations, and b) at least some of the normal faults were initiated at a low-angle and are not the

result of later rotation (Morley, 2009; Fig. 7).

The widely accepted models of LANF development discussed by Buck (1991) and Whitney et al.

(2013) require the crust to be warm or hot, and possibly over-thickened to generate low-angle normal faults

(Fig. 8). Whitney et al. (2013) view all the origins of LANFs as variations on rotation of high-angle faults to

a lower angle (Fig. 8). It is certainly probable hot, thickened crust existed in Thailand during the Cenozoic

as a consequence of an oblique Andean-type margin setting during the Late Cretaceous and Palaeogene (e.g.

Morley, 2004, 2012; Searle et al., 2007; Palin et al., 2013), and that the rift basins are in part related to col-lapse of this thickened crust (Morley et al., 2011; Morley, 2012). But that does not have to mean that all the

low-angle normal faults originated as rotated high-angle faults following the models reviewed by Whitney et

al. (2013; Fig. 8). A key conclusion by Morley (2009) was that at least in some cases (Doi Inthanon-Chiang

Mai Basin, Phitsanulok Basin, Suphan Buri Basin) the LANFs originated as low-angle faults and were not

rotated higher-angle faults. The development of the footwall areas, and relationships of the faults to the sedimentary basins simply did not fit a rotating high-angle normal fault model (Morley, 2009; Fig. 7). The

cause of the low-angle faults is probably that they followed weak, low-angle, east-dipping compressional fabrics generated during the Triassic-Early Jurassic Indosinian orogeny (Morley, 2009). The LANFs do not

represent a single stage in the rift development, the Rayong and Khanom areas may represent early collapse of the crust at the onset of extension during the Eocene (Morley et al., 2011), and are the best candidates for

rotated, initially high-angle normal faults. Other basin-bounding faults were not active until the Late Oli-gocene or Early Miocene and are the ones that initiated at a low-angle (e.g. Chiang Mai Basin, Phitsanulok

Basin, Morley, 2009).

The impact of LANFs on hydrocarbon prospectivity is two-fold: 1) as a sediment source area during

unroofing (e.g. Chiang Mai Basin), and 2) the influence on depositional systems (e.g. Phitsanulok Basin) and

particularly the source/seal-reservoir relationships found in the Sirikit Field. Examples are discussed below.

Footwall denudation The Chiang Mai basin is a Late Oligocene-Miocene rift basin about 125 km long, and 25 km wide

(Fig. 3). It is bounded on its western margin by a large, low-angle normal fault system (Dunning et al., 1995;

Rhodes et al., 2000). Mid-crustal level ortho- and para-gneisses are exposed in the footwall of fault LANF

(Dunning et al., 1995; Macdonald et al., 1993, 2010; Fig. 9). Mica Ar-Ar and apatite fission track ages indi-cate that unroofing of this metamorphic core complex occurred between about 21 and 14 Ma (Upton, 1999;

Morley, 2009). Making some simple assumptions about the depth to the top of the unroofed amphibolitic

gneisses at the start of extension (10 km depth), and the amount of extension (up to 35 km, Morley, 2009),

the total volume of material eroded or translated eastwards to achieve the unroofing is about 15,000 km3, of which the volume of eroded crust is estimated at around 7,000 km3. The cooling ages indicate this material was removed over a period of about 11-13 my. The total volume of sediments filling the Chiang Mai basin

estimated from seismic reflection data is about 5,000 km3, and perhaps represents deposition over a 15-20

million year period. Consequently the amount of material eroded from the Doi Inthanon/Doi Suthep area

far exceeded the capacity of the Chiang Mai Basin during the Early Miocene. Curiously, Miocene alluvial fan deposits on the west-side of the basin, with one exception, do not contain clasts of gneiss from the Doi

Inthanon source area (Rhodes et al., 2005; B. Rhodes pers. comm., 2012). This suggests that while the basin

was being transported east the gneisses were not widely exposed, and that by the time the gneisses were exposed and eroded the basin was lacking accommodation space. The Chiang Mai Basin LANF created a lo-cal sediment source area that generated a volume of sediment that far exceeded the capacity of the adjacent Chiang Mai Basin. Consequently a pulse of sediment coming from the Doi Inthanon/Doi Suthep area during

the Early Miocene would have worked its way into the rift basins further south. Provenance studies to inves-tigate whether reservoir sands in basins further south can be linked with the unroofing of the Doi Inthanon

area have yet to be conducted. Footwall uplift associated with high-angle extensional fault systems will typically be a much smaller source of sediment supply than the Doi Inthanon/Doi Suthep example. The footwall is clearly the source

area for many boundary fault margin fan systems in rifts, but such systems are typically confined to a few

kilometres width from the boundary fault. These fans do not represent a high volume of sediment input compared with drainage systems that enter from the axial and flexural margins. The East African Rift clearly

shows a strong link between faulting and footwall topography (e.g. Hendrie et al., 1994; Kusznir et al., 1995;

Sachau and Koehn, 2010), although uplift at the largest scale is associated with mantle upwelling effects

(Ebinger et al., 1989; see reviews in Pik, 2011 and Moucha and Forte, 2011). Flexural cantilever modeling

of the rifting also highlights the potential for several kilometres of footwall section to be removed by erosion during extension (e.g. Hendrie et al. 1994; Kusznir et al., 1995;). Yet, a regional apatite fission track study in

Kenya identified several episodes of denudation and cooling during Cretaceous-Palaeogene times, but rela-tively little basement denudation during the Miocene-Recent period of rifting (Foster and Gleadow, 1993,

1996). Only using the (U-Th)/He technique, which is sensitive to smaller amounts of denudation and cooling

than apatite-fission track, has a younger cooling event during the last 5-10 Ma been identified (Spiegel et al.,

2007). Hence, the denudation associated with Doi Inthanon is considered anomalous compared with typical

continental rifts.

Sirikit FieldThe Phitsanulok Basin (Figs. 3 and 10) in Central Thailand is an example of a simple rift. Many of

the numerous fields in the basin are typical simple rift plays, with relatively low-displacement faults setting

up small traps, where only a few of the numerous potential sandstone reservoirs are laterally sealed and trap hydrocarbons. One field, however, stands out as being unusually large. Sirkit Field is a tilted fault block trap

(Fig. 11), apart from its setting in a continental basin, in terms of structural style and size (~14 km2), it is a

good match for a typical North Sea-type tilted fault block trap (Morley et al., 2007b). However, typically

in the North Sea the Early Cretaceous post-rift section is an important element for the top seal. About 200

million bbl oil have been produced from the Sirkit Field, and the recovery factor is relatively low at about 16%. The Sirkit Field represents the perfect storm of simple rift traps in a continental rift depositional set-ting. The key to its size is that an axial fluivio-deltaic system prograded from the north over the top of the

tilted fault block (Fig. 12). Successive transgressions and regressions resulted in a sequence of alternating

sands (Lan Krabu Formation) and shales (Chum Saeng Formation), that juxtaposed source/seal rocks and

reservoir rocks over a structural high. Growth of the structural high was episodic, and insufficient to cause

complete erosion of section at the fault block crest (Morley et al., 2007b). The shales were sufficiently thick

to also be smeared into the main fault zone and hence to play an important role as both lateral and top seals. The unusual feature of the basin is that the main hydrocarbon kitchen area, and main depocentre (Sukhothai

Depression) lies north of the Sirikit Field. Hence, it would normally be expected that high subsidence along

the bounding fault would prevent the southward axial progradation of the fluvio-deltaic system from reach-ing Sirikit Field. What makes this setting anomalous is the presence of the very low-angle Western Bound-ary Fault, which dips at about 25 E, has a maximum heave in excess of 12 km, and lies on the western side

of the Sukhothai Depression (Morley et al., 2007a; Morley, 2009).

Since the amount of (vertical) throw on a fault is related to the amount of extension (heave) and fault

dip angle, assuming similar strain rates high-angle faults will create more accommodation space than low-angle faults for a particular time-interval. Hence, the potential for high-angle faults outstripping sedimenta-tion rates, and creating sediment starved basins, and locations for large deep lakes, is considerably greater than for low-angle normal faults (Morley, 1989). Despite 12 km heave on the Western Boundary Fault, the

low throw:heave ratio meant that the hangingwall depocentre was at least frequently and temporarily unable

to keep up with sedimentation rates, hence the fluvio-deltaic sediments prograded over the entire depocentre

(Fig. 12). After an initial pulse of lacustrine sedimentation, the Sukhothai Depression was subject to pre-dominantly fluvio-deltaic deposition, and lacustrine conditions remained south of the Sukhothai Depression,

where smaller, but higher-angle faults were able to sustain lacustrine conditions (Fig. 12). The Sirkit Field

tilted fault block lay in the perfect location for this transition. Had the Sukhothai Depression been bounded

by a large high-angle normal fault it is likely the fluvio-deltaic deposits would have remained confined to the

Sukhothai Depression, and never reached the Sirikit Field area depriving it of good reservoir rocks.

Basin InversionIt is generally recognized within the petroleum industry that locally inverted rifts tend to enhance hy-

drocarbon prospectivity, while regional inversion tends to degrade it (Macgregor, 1995). In SE Asia, two of

the largest hydrocarbon provinces: the Central Basin, Sumatra, and the Malay Basin, offshore East Malaysia, have the great majority of their reserves within inversion-related traps in the post-rift section (Macgregor,

1995). The timing of inversion when it affects rift and post-rift basins is usually either shortly after the end

of rifting, or at any time during or after the post-rift subsidence phase depending upon how the tectonic con-ditions of the area have evolved. What is atypical is to find examples of rifting alternating with inversion, yet

this is the case for some rift basins in Central and Northern Thailand.

The distribution of inversion in rift basins in Thailand is highly variable. As mentioned above, the Malay Basin is strongly inverted, but passing northwards into the North Malay Basin and Gulf of Thailand

inversion is much less important. There are, never-the-less, examples of inversion around the Oligocene-

Miocene boundary (the end of the early rift phase in the gulf) in a few basins (Chumphon, Khmer Basin,

Mergui Basin; Morley et al., 2011). In the NW gulf, minor inversion occurs at the end of rifting during the

Middle Miocene. While in a number of basins, particularly onshore, inversion occurs at the end of rifting

around the Miocene-Pliocene boundary. Only in the Fang Basin (Pliocene inversion) in northern Thailand

have any of these events provided traps for hydrocarbons (Racey, 2011). Due to the diachronous nature of

the rifting events discussed in the section Diachronous Termination of Rifting, the Middle Miocene and

Pliocene inversion structures can be classed as the end of rifting-type structures. The more unusual inversion structures occur in Central and Northern Thailand. Morley et al. (2000)

noted the possibility of 4-5 inversion events in the Late Oligocene-Miocene Li Basin based on outcrop

observations. Such inversion events have impacted the Phitsanulok Basin, which is the most prolific onshore

hydrocarbon basin. The northern half of the basin contains the Sukhothai Depression and Sirkit Oil Field

(Fig. 10). Inversion structures in the northern half of the basin occurred at the end of rifting, at the Miocene-

Pliocene boundary (Morley et al., 2007a, 2011). The southern half of the basin has no hydrocarbon discover-ies. The basin can be sub-divided into the Lahan, and Nong Bua and Dong Chat sub-basins (Fig. 10). The

Lahan Graben comprises Oligocene-Early Miocene section. Subsidence in the basin ceased after that time,

which is different from the Sukhothai Depression to the north and the Nong Bua sub-basin to the East, both

of which continued to subside within fault-bounded half graben during the Middle and Late Miocene. The cessation of subsidence in the Lahan Graben is marked by strong inversion in a transfer zone region between the Lahan Graben and the Nong Bua sub-basin (Fig. 10). Two phases of inversion affect the transfer zone

region (Fig. 13). The first inversion event created widespread folding, and erosion, which is marked by an

angular unconformity. The second inversion event is more subtle, and is seen on one fault, where the brown unit is thinner in the hangingwall than in the footwall (Fig. 13B, C). One change further north, potentially

related to stress changes associated with inversion, is that the extensional fault pattern is re-organized from more N-S trending faults to NE-SW trending faults about the same time as the inversion (Morley et al.,

2007a).

Inversion of the syn-rift section has not produced any large closed structures, and effectively marks

the end of significant basin subsidence, and the termination of hydrocarbon maturation in the basin. Prob-ably the single largest contributing factor to the absence of any hydrocarbon discoveries in the southern half of the Phitsanulok basin is the Middle Miocene inversion. Extension during the Middle and Late Miocene was episodic in Central and Northern Thailand.

Most of the rift basins are not very deep, and the basin fill may amount to a few hundred metres (e.g. Mae

Moh and Li basins), or in the deeper basins (Chiang Mai, Lampang, Phrae) the stratigraphy is just not well

dated (Morley et al. 2001). Only in the Sukhothai Depression is the Middle-Late Miocene stratigraphy well

known (in terms of numerous well penetrations, and seismic coverage). This time period comprises the

uppermost part of the Pratu Tao Formation, the Yom and Ping formations (Flint et al. 1988; Morley et al.,

2007a). The three formations combined commonly exceed 1 km thickness. These are alluvial plain to al-luvial fan deposits, and are consequently difficult to date, and contain numerous minor erosion surfaces and

periods of non-deposition. Hence episodes of rift quiescence that correspond with periods of inversion in the

southern part of the basin could easily be hidden within this type of poorly dated, continental stratigraphy. Post-rift subsidence The Malay, North Malay, and Pattani Basins are characterized by a Late Eocene(?) to Oligocene syn-

rift section and a Neogene post-rift section. The post rift section in the Pattani Basin exceeds 6 km thickness

in some places, and 8 km in the Malay Basin (Morley and Westaway, 2006; Fig. 14). Both the thickness and

subsidence rates are much greater than failed rifts with sag basins such as the North Sea and Anza Graben

(part of the Central African Rift System in Kenya), (Fig. 15). Note that the Anza Graben shows large, rapid

subsidence in the syn-rift phase, but minor subsidence in the post-rift phase. Also, for comparison subsid-ence curves from three classes of basin (data from Xie and Heller, 2009) are shown, plotted at the same scale

as the other basins. The only basins that compare in terms of subsidence rate are strike-slip basins, but it is important to emphasize that the post-rift subsidence patterns of the Pattani and Malay basins are not fault controlled, they are deep, saucer-shaped basins that have subsided rapidly over a broad area. In the Pattani

Basin minor normal faults are developed (but they are clearly not basin-bounding faults), while in the Ma-lay Basin inversion anticlines grew during post-rift subsidence. The subsidence is also associated with high heat flow (up to 110 mWm2) and geothermal gradients (up to 6C/100 m), (Morley and Westaway, 2006).

Consequently despite being about 23 my into the thermal subsidence phase, the basins are associated with

temperatures more typical of rift basins than post-rift basins. One way to try and explain the high subsid-ence rate is to begin the post-rift phase with a large accommodation space that needs filling (i.e. considerable

water depths), but that is impossible for these basins, the syn-rift fill is continental, while the post-rift fill is

predominantly fluvio-deltaic to marginal marine. Deposition has always been close to sea level, and even

today the greatest water depth in the Gulf of Thailand is only 80 m. The basin depositional style is predominantly aggradational, and subsidence has kept pace with sedimentation during deposition. Consequently the basins superficially look as if sediment loading has

driven basin subsidence. However, basic isostatic principals dictate that sediment loading cannot drive basin subsidence to the extent required. As reviewed by Morley and Westaway (2006) the main explanations are

either extreme syn-rift crustal extension (Madon and Watts, 1998), or crustal flow away from the basinal

area during post-rift subsidence. Huismans and Beaumont (2008) discussed a general model for passive margin development, where

some basins were undergoing sag basin formation, while adjacent basins underwent syn-rift, fault controlled subsidence. Possibly a similar model in the Gulf of Thailand could be applied where crustal flow from

beneath the Pattani and North Malay basins was triggered by flow into the actively extending areas (Fig.

2). Gravity data over the Gulf of Thailand shows the Pattani and North Malay basins have a small negative

gravity anomaly, while the basement high areas have a larger negative gravity anomaly (Milsom, 2011), i.e.

uplift of the mantle (thinning of the crust) beneath the rifts has been so large as to dominate the expected

large negative gravity anomaly associated with a thick sedimentary basin. The opposite situation character-izes the western gulf, where large negative anomalies are associated with the sedimentary basins, and indi-cate crustal thinning is moderate (Milsom, 2011). Crustal flow to the west could help explain the pattern of

gravity anomalies in the gulf. One effect of the rapid post-rift subsidence is the creation of high overpressures in the thicker parts

of the basin that can be a drilling hazard. The overpressure, which can attain a pore fluid pressure ratio of up

to 0.9, appears to be caused not only by disequilibrium compaction, but also by gas generation within coal-

rich sequences within the Middle Miocene (Tingay et al., 2013).

Post-rift extensional faults

On seismic reflection data, the syn-rift section of the Pattani Basin shows a clear half-graben style,

with predominantly N-S trending faults (Watcharanantakul and Morley, 2000; Morley and Westaway, 2006).

The North Malay basin shows a mixture of NW-SE and N-S trending faults syn-rift faults that bound half

graben (Morley et al., 2011). In the deepest parts of the basins the post-rift section is sufficiently thick that

the syn-rift is not imaged on 6 seconds two way travel time seismic data. The major syn-rift faults can have throws of several kilometres. There are no faults of such magnitude that affect the post-rift section; how-ever, anastomosing zones of low-displacement normal faults are very well developed. A seismic reflection

package characterized by continuous, high amplitude reflections forms a distinctive zone that separates the

syn-rift and post-rift sections. Known as the K-shales, this zone corresponds with Late Oligocene-Early

Miocene organic-rich lacustrine shales. Most of the faults visible on seismic data have throws in the order of 10s metres, with the very infrequent, largest faults showing displacements up to about 400 metres. The

faults tend to form convergent dipping conjugate sets. These small faults set up numerous, but small traps, where predominantly channelized sands trending at a high angle to the fault zone are laterally sealed by the fault (Rigo de Rhigi et al., 2003; Racey, 2011). The sands are predominantly filled with gas or gas and con-densate. The sands are stacked against the fault, and gas filled sands can alternate with sands with low gas

saturations, or water filled sands. Alternations of gas and water bearing sands can occur seven or eight times

in a well. The timing of fault activity is highly variable, with some faults terminating close to the sea floor,

while others ceased activity during the Miocene. Reactivation of the faults is a significant factor in breaching

traps and leaking fluids. Evidence for this fluid movement is seen in the mixing of gases of different ther-mal maturity in traps along the fault, and in the wide distributions of gas saturations in adjacent reservoirs. Rarely are reservoirs filled to spill point.

Faults imaged on 3D seismic data in the post-rift basins of Thailand provide excellent examples

of multiple fault linkage geometries (Kornsawan & Morley 2002; Rigo de Rhigi et al. 2003; Morley et al. 2004). Contour patterns of the faults frequently show displacement maxima in the post-rift section, with dis-placement dying out downwards as well as upwards, indicating that the faults nucleated within the post-rift section (Kornsawan & Morley 2002; Morley et al. 2007a) (Fig. 16). Some syn-rift faults have propagated upwards into the post-rift section and there are examples of vertical linkage between syn-rift and post-rift faults (Fig. 16a, b). Very long, low displacement faults are present in the post rift section (Fig. 16d,e). In the

Pattani Basin these faults tend to strike N-S, whilst in the North Malay Basin they strike NW-SE (Morley

et al. 2004) (Fig. 17b,e). These long-low displacement faults tend to link and align along underlying syn-rift faults and give rise to a long fault with multiple along-strike displacement highs and lows indicative of previously isolated faults that have linked (Figs. 16b1, 17d,e). Some of these faults are 40-80 km long and

may be composed of 20-30 linked faults that were initially 1-4 km long, and may have maximum throws of

only 100-300 m. This mixture of low-displacement short and long faults produces a considerable spread of points on a length-displacement graph (Fig. 18). The fault pattern is particularly well developed in the North

Malay Basin where short N-S striking fault segments curve to join long NW-SE trending faults (Leo 1997;

Morley et al. 2004). The long faults display significant under-displacement for their length (length: displace-ment ratios up to ~ 300:1), when compared with typical fault sets worldwide, (length: displacement ratios

around 10-20:1, e.g. Walsh & Watterson 1988; Dawers et al. 1993) (Fig. 18). For comparison the syn-rift fault displacement-length data from the Mae Moh Mine (Northern Thailand) is plotted with the North Malay

Basin data (Fig. 18). The syn-rift faults display scatter in displacement: length ratio due to fault linkage, but

clearly the post-rift faults show much lower length: displacement ratios than the syn-rift faults. The faults of the Pattani and North Malay basins represent an unusual style and occurrence of fault.

Unusual because they are systematic, low-displacement faults developed in a post-rift basin. The style is

atypical because the faults are conjugate in cross-section, but whereas ideal conjugate faults have sub-paral-lel trends in map view, the post-rift fault sets are commonly composed of two or three important directions due to underlying syn-rift fabrics or local rotation of stresses (Morley et al., 2004; Tingay et al., 2010). Di-rections related to pre-existing (syn-rift) faults tend to give rise to very long, low-displacement faults. These

faults have trapped gas in very numerous, but small accumulations. Initially the accumulations were consid-ered non-commercial, but Unocal were able to exploit them by the early application of 3D seismic technol-ogy to place deviated wells on stacked reservoir targets within 70-170 m of the fault, and by rapidly drilling

about 150 development wells per year, at very low cost (~0.8 million USD/well; Valusek, 1998).

The author is unaware of other examples of post-rift basins with the conjugate fault style displayed by the Pattani and North Malay Basins. The faults have developed during the Middle Miocene to Present,

but the causes of their formation and activity remains uncertain. Tingay et al. (2010) suggested that the faults

formed episodically in response to strain imposed on the Gulf of Thailand from large earthquakes on the

Sumatra-Andaman subduction zone. This interpretation follows from observations of the considerable strain gradient across the gulf arising from co- and post-seismic motions associated with the 2004 Sumatra earth-quake. Whether this is a feasible mechanism remains to be determined.

Tectonics underlying the anomalous characteristics The rift basins of Thailand are failed rifts within continental crust, but they lie within a distinctly different tectonic setting from the failed rifts used for comparison in this paper. The Gulf of Suez, North

Sea, Central African and East African rifts all lie within old stable continental crust and all, except the Gulf

of Suez (failed propagating tip), are at least in part aulacogens branching off passive margins. In contrast,

Sundaland is surrounded by subduction zones, and has also undergone the collision of India with Eurasia,

and Australia with Indonesia. The Sundaland crust began growing by accretion of various terranes during the

Indosinian orogeny in the Triassic, and the process is continuing today (Hall, 2012). Hence, the crust is full

of relatively young heterogeneities at a variety of scales. In many places it is heavily intruded by Triassic-

Eocene granitic plutons that contribute radiogenic heat both to the crystalline crust, and radiogenic sedi-ments to basins (Morley and Westaway, 2006). Subduction processes also enhance the mantle heat flow into

the crust (Hyndman et al., Hall and Morley, 2004). Hence, in many places Sundaland crust is hot and weak

and consequently tectonics are much more dynamic in than in the other regions of failed rifts (Hall, 2011).

Conversely, there is no indication of mantle plume activity affecting Thailand. The tectonic setting feeds back into the five anomalous features discussed above in a number of

ways. The highly variable location and timing of inversion, and the varied onset and termination of rifting reflects a rapidly evolving stress field both temporally and spatially. The modern stress field, unlike most

continental areas is not dominantly aligned with the maximum horizontal stress parallel to absolute plate motion (Richardson, 1992). Instead it is distinctly mis-aligned (Fig. 19; Tingay et al. 2010b ). This mis-

alignment is a consequence of forces arising from multiple plate boundaries, plus intraplate sources of stress,

which have produced a more complex stress pattern than is typical of continental areas. Variations in stress magnitude and orientation as the plate boundaries have evolved can help explain why alternation between extension and inversion is so varied (e.g. Hall and Morley, 2004; Pubellier and Morley, in press).

The low-angle normal faults may reflect both the hot, weak, nature of the crust, and the effects of

pre-existing fabrics related to the collision history. The rapid post-rift subsidence can be related to two key factors, a rapidly eroding sediment source area (largely local to Thailand; Hall and Morley, 2004), which in

particular reflects the tectonically active western region (Shan Plateau area), and also hot crustal conditions

that were conducive for rapid subsidence perhaps in response to lower crustal flow entirely related to post-

rift subsidence (Morley and Westaway, 2006), or where post-rift subsidence in the Pattani and Malay Basins

is related to crustal flow in response to non-uniform extension in adjacent active rift basins.

Conclusions The intra-cratonic, supra-subduction zone setting of the Thailand rift basins have caused the rifts to evolve in a number of ways that are different from other intra-cratonic rifts developed in older continental lithosphere. The key differences focused on in this paper are the widespread occurrence of low-angle nor-mal faults, mixed with higher-angle faults; basin inversion alternating with rifting; diachronous initiation

and cessation of rifting; rapid post-rift subsidence and the presence of extensive, low-displacement post-rift fault. These characteristics are related to the presence of hot, weak continental lithosphere, a rapidly evolv-ing stress regime due to the rapid evolution of the plate boundaries during the Cenozoic, and the history of subduction and accretion (Fig. 20).

The impact of tectonics and the resulting structural and depositional characteristics is summarized in Figure 20. Low-angle normal faults in general reduce the likelihood of source rocks developing, but in the

Sirikit Field resulted in fluvio-deltaic reservoirs interfingering with the northern margin of a lake controlled

by higher-angle faults. Basin inversion alternating with syn-rift section has been detrimental to the prospec-tivity of the southern half of the Phitsanulok Basin by destroying syn-rift structure, and not providing any significant closed, inversion related structure, and halting hydrocarbon maturation early in the basin history.

The diachronous timing of basin development has been good for the eastern Gulf of Thailand basins where a long history of rapid post-rift subsidence has resulted in deposition and maturation of source rocks, and kept depositional facies belts quite stable, and aggradational in the post-rift section. Diachronous timing has not

been good for the younger basins that lack the post-rift section, and the greater variety of petroleum systems and traps that post-rift basins offer. The origin of the post-rift faults is uncertain, but they are influenced by

pre-existing fabrics, and are in rapidly subsiding basins, which are probably a consequence of hot, weak

crust. They are the most significant hydrocarbon traps in the Pattani and North Malay basins, and provide

important migration pathways for fluids in the basin. They represent a structural style not usually found in

post-rift basins.

AcknowledgementsI would like to thank PTTEP for funding and data. Mark Tingay and Ian Watkinson are thanked for helpful

and constructive reviews.

FiguresFigure 1. Regional location map showing the main Cenozoic basins and tectonic features. Modified from

Searle and Morley (2011).

Figure 2. Regional cross-section through the southern Thailand rift basins, see Fig. 1 for location.

Figure 3 Regional map of Thailands Cenozoic basins and structures. Modified from Morley et al. (2011).

Figure 4 Generalized charts of Cenozoic structural activity in key rift basins in Thailand.

Figure 5. Schematic illustration of the modes of aulacogene development, A1 and A2), propagation inland

of the continental rift system (e.g North Sea); B) Simultaneous development of inland rift associated with

mantle plume, with a second mantle plume at the triple junction (e.g. Eastern Branch of East African Rift);

C) Simultaneous development of isolated rift basins along the length of the aulacogene, followed by small

local propagation to link up the basins (e.g. Central African Rift System).

Figure 6 Recoverable reserves for three categories of rift-related hydrocarbon province, A = simple rifts, B = rifts with post-rift sag basins, and C = passive margins. Some provinces on passive margins were excluded because they are much more related to large deltaic systems, independent of the syn-rift setting (these in-clude the Nile and Niger Deltas and the Gulf of Mexico). West Siberia is excluded from post-rift sag basins

because it is such a large, and anomalous basin, and rifting may not be the only significant driver of basin

formation. Chart A = present known produced and unproduced reserves. Chart B = chart A plus predicted P50 undiscovered resources, the changes largely reflect future deepwater exploration of the Atlantic passive

margins and how that may increase the relative contribution of passive margins to the total amount of hydro-carbons discovered in rift-related basins.

Figure 7 Example of a large, low-angle normal fault from the Mergui Basin. The fault plane is characterized

by high amplitude reflections down to a depth of about 12 km. The fault plane is low-angled and relatively

straight for most of its length. It does not appear to have been rotated significantly.

Figure 8 Models for the development of low-angle normal faults in response to different initial crustal condi-tions (temperature, thickness) according to Whitney et al. (2013).

Figure 9 Regional cross section from the Chiang Mai Basin across Doi Inthanon, illustrating the main struc-tural and stratigraphic features, based on outcrop and seismic reflection data, and structural relationships

described by Dunning et al. (1995), Barr et al. (2002) and Morley (2009), apatite fission track ages from Upton (1999). See Fig. 3 for location. b. Regional crustal-scale section across northern Thailand, the deep crustal structure is mostly inferred from Bouguer gravity data. c. Sketch of the tectonic setting of central and northern Thailand during the early Jurassic, at the end of the Indosinian Orogeny. The approximate location

of section a) is located to illustrate the origin of the key rock units (Sibumasu para- and ortho- gneisses), Pa-

laeozoic rocks above the Lower Detachment represent the Inthanon Zone. Redrawn from Sone & Metcalfe

(2008).

Figure 10 A) General structural map of the Phitsanulok Basin. B) Structural sketch map of inverted transfer

zone in southern part of the Phitsanulok Basin.

Figure 11 Cross-section through the Sirikit Field (see Fig. 10 for location). Based on Morley et al. (2007b).

Figure 12 Paleogeography of the Phitsanulok basin during the Late Oligocene-Middle Miocene, largely based on unpublished maps made by Thai Shell in 1988. Maps based upon well data, supplemented by 2D seismic where sedimentary environment can be inferred from seismic character. Outlines of major faults are shown. Figure 13 A1), B1) Uninterpreted, and A2), B2) interpreted seismic lines showing inversion structures in the

southern Phitsanulok Basin. See Fig. 10B for location.Figure 14 Cross-section through the Malay Basin, redrawn from Madon (2007).

Figure 15 Comparison of subsidence rates from different basin types (based in part on Xie and Heller, 2009,

and Morley and Westaway, 2006).

Figure 16 Schematic illustration of the way post-rift faults in the Pattani and North Malay basins interact.

a1, b1, c1 diagrams of displacement contours on fault surface, for fault i, darker grey indicates higher displacement. a2, b2, c2 cross sections of post-rift conjugate fault systems, most of the faults have nucle-ated within post-rift section, but in cases a and b fault i has a relationship with syn-rift faults. a1. Fault i is a syn-rift fault that has reactivated during the post-rift stage and propagated upwards into the post-rift section, displacement dies out upwards and laterally. a2. cross-section showing large syn-rift offset on fault i and much smaller post-rift offset. b1. Several low-displacement faults have nucleated within the post-rift section but have been influenced (in location, dip and strike direction) by the deeper syn-rift fault (probably

due to local stress rotation) and linked with it. The single large displacement maxima on the syn-rift fault is

overlain by multiple, smaller magnitude displacement maxima in the post-rift faults. The map view displays of fault patterns a,b1 and a,b2 are labelled to indicate that both geometries can occur for cases a and b. a,b1. The syn-rift fault lies parallel to the regional extension direction because it is a large-displacement syn-rift fault it is much longer than the lower-displacement post-rift faults. This is reflected in the length

of fault i. Note the conjugate faults tend to undergo systematic changes in dominant dip direction passing

along strike, this produces a series of transfer zones where the tips of the opposite-dipping faults slightly overlap. In the Gulf of Thailand these features are informally called graben shifts. a,b2. the syn-rift fault trends obliquely to the post-rift stress field, consequently fault i forms a long oblique fault trend relative to

the shorter post-rift faults. Post-rift faults with approximately similar dip direction to the syn-rift fault tend to curve and join it. Post-rift faults dipping in the opposite direction cannot join the syn-rift trend, hence tend to be short, isolated faults. c1. Fault i is one of a number of similar post-rift faults with no link to any syn-rift structure, displacement maxima is in the post-rift section and the fault dies out in all directions from the centre. c2. A basement high nucleates the occurrence of the post-rift conjugate faults, no syn-rift faults are present. The faults die out downwards. c3. Conjugate normal fault sets, with no dominant fault trend, reflect-ing the absence of long syn-rift faults. From Morley et al. (2011).

Figure 17 Illustration of syn-rift and post-rift fault patterns in the eastern Gulf of Thailand. a) Regional syn-

rift fault map of the Pattani and Khmer basins based on 2D and 3D seismic reflection data, illustrating fault

patterns in the Late Oligocene-Early Miocene syn-rift section, and b) Detail of post-rift fault swarm patterns

from the Northern Pattani basin (redrawn from Rigo de Rhigi et al. 2002). c) The post-rift faults tend to form

curvi-linear trends of convergent conjugate faults with 10s meters to a few hundred metres displacement. These trends often appear to be guided by underlying syn-rift faults which localize long, low-displacement post rift faults, as illustrated for the Tantawan graben. d) Time structure map fo the Tantawan graben (redrawn

from Rigo de Rhigi et al. 2002), absolute scale not shown (darker colours = deeper time-depths). Many of the

post-rift faults tend to have different strikes from the underlying syn-rift faults but join, or splay off trends controlled by the syn-rift faults. e) Example of fault map view geometry in the post-rift section from the North

Malay basin (Morley et al. 2004). In d) and e) the black faults dip E to NE, while the light grey faults dip to

the W to SW. (From Tingay et al., 2010a).

Figure 18 Length-displacement graph for post-rift faults in the North Malay Basin (faults mapped from 3D

seismic reflection data), compared with plots for syn-rift faults in Mae Moh mine (Morley and Wonganan

2000).

Figure 19 Distribution of misfit between maximum horizontal stress orientations and absolute plate motion

in the Sunda Plate (A-C quality in light grey, D quality in dark grey. From Tingay et al. (2010a).

Figure 20. Relationships between tectonic setting, structures and the hydrocarbon system of the Thailand

rift basins.

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10

0 100 km

20

30

Dep

th (k

m)

0

Sewell Rise

East Andaman Basin

Mergui Basin PeninsularThailand

Songkhla Basin Pattani Basin Khmer Basin

Lower Crust

Upper Crust

W E

Flow of crust?

Fig. 1

Fig. 2

1

MALAYSIAN PENINSULA

?

? ?

?

500 km

Andaman Sea

INDIA

Aliao Shan-Red Red

River Shear Zone

Spratly Rise-DangerousGrounds

W. Natuna Basin

N.SumatraBasin

South China Seas oceanic crust

Borneo Lupar Line

Cuu Lon

g Basin

Palaw

an Tr

ough

Mae Ping Fault

Mergui Basin

Sumatran Fault

Baram Line

China Block

Indo-Burma Ranges Bengal Basin

Khorat Plateau

Qui Nhong Ridge

Gulf of Tonkin Basin

Lanping-Siamo Fold Belt and Khorat Plateau

Central Basin

Gulf of Thailand

Java

Sumatra

Sunda Shelf

Three Pagodas Fault

Himalayas

IYS

Malay basin

Kutai Basin

TarakanBasin

Approximate area of basin

Bengal Basin (foredeep basin)

HT = Himalayan TerraneLCT = Lhasa-Coqen TerraneTB = Tanggula BlockIYS = Indus Yarlung SutureMetamorphic core complex (MCC) (Late Oligocene-Early Miocene)

DI- DS

M = Mogok MCC, 26-21 MaDI-DS = Doi Inthanon-Doi Suthep MCC

HT

LCT TB

Tibetan Plateau rifts and metamorphic core complexes

Fig. 1

Saga

ing

Faul

t

GPS derived velocitywith respect toSundaland(Simons et al. 2007)

20 mm/yr

Hoang Sabasin

Andaman-Sumatra

Trench

90

105

15

30

0

Fig. 2

Sundaland GPS station

Approximate outline of Sundaland

M

Dai Nui Con Voi

Pearl Rive

r Mouth

Basin

?

East Andaman-

South

?

Bangkok

Chiang Mai Basin

Hua-Hin-Pran BuriMylonites,northernRanong Fault Zone

Possible early Cenozoicmigmatites in gneissesexhumed along theThree Pagodas Fault Zone

Mae PingFault Zone

DI

Rano

ng F

ault

Klon

g Mar

uiFa

ult

Three PagodasFault zone

Mae ChanFault

Ranong Faultmylonites, Khao PLaiKhlong Hin Phao

Khao Phanommylonites

Khanom

6oN-

8oN-

10oN-

12oN-

14oN-

16oN-

18oN-

l98oE

l100oE

l102oE

F CR

KU

CHP NPA

CPA

SO

NK

K

WE

PU

N

CRP

CM

PET

SKN

LP

NB

NS

LY

SBAY

TB

PNSK

KS

KB

LI

EKNW

PR

SPA

KH

HH N

arat

hiw

at H

igh

Ko Samui High

MS

MSR MM

NYP

ST

PH

PHR

P

MC

ML

MLA

Cambodia

Malaysia

Myanmar

Laos

Thailand

TH

200 km0 100

WH

WH

PT

AS

GOT

KP

CB

NCB

WH = Western HighlandsNCB = North Central RegionCB = Central BasinKP = Khorat PlateauGOT = Gulf of ThailandPT = Peninsular ThailandAS = Andaman Sea

Uttaradit Fault Zone

Mer

gui R

idge

Cenozoic strike-slip or normal fault

DI = Doi Inthanon

Deformed granite-gneiss that maydisplay Cenozoicmetamorphism andmigmatitesCenozoic folds inKhorat Group

FCR

PU

KU

N

PH

CRP

CM

PETSKN

LP

NB

NSLY

SBAY

TB

PNSK

KS

NWHH

Chiang RaiFangPuaChiang Rai-PayaoNanChiang Mai

PhitsanulokLampang

Sakhon NakhonPhetchabunNong BuaKhorat UdonLad YaoNakhon SawanSuphan BuriAyutthayaKamphaeng SaenThon BuriSakhonPaknamHua HinN. Western

CHPPRKH

NPACPASPA

MASO

NK

KEK

WE

MER

WesternKraEast KraNorth PattaniCentral PattaniSouth Pattani

ChumphonNakhonMerguiSongkhla

KBMerguiKrabi

LI Li

Khmer Prachuap

NMB North Malay BasinSurat Thani Nong y PlongEast Andman

Mae MohMM

MSMSR

NYPEA

ST

PhraeMae SotMae Sariang

PHR

PiPMC

MLMLA

Mae Chaem

TH Thoen Basin

Mae LamaoMae Lai

Cenozoic basin

MER

EA

NMB

Outline ofChao PhrayaBasin

Khao TaphaoKhwam (low-anglenormal fault

Fig. 3

N-S Ch K We SB Ph Chi/L MM CMFNM Kh

10

0

20

30

40? ? ? ?

Post-rift basin Syn-riftInversion

Central North CentralWestern GOTEastern GOT

Pa

Period ofrapid post-riftsubsidence

Unconformity

Ma

Pliocene-Recent

Mio

cene

Olig

ocen

eEo

cene

Late

Late

Late

Middle

Middle

Early

Early

Transitional syn-rift to post-rift

A1

A2 B C

Mantle plume

Mantle plume

Triple junction

Spreading centre

Passive m

argin

Aula

coge

n Rift propagation direction

Fig. 4

Fig. 5

South

?

Bangkok

Chiang Mai Basin

Hua-Hin-Pran BuriMylonites,northernRanong Fault Zone

Possible early Cenozoicmigmatites in gneissesexhumed along theThree Pagodas Fault Zone

Mae PingFault Zone

DI

Rano

ng F

ault

Klon

g Mar

uiFa

ult

Three PagodasFault zone

Mae ChanFault

Ranong Faultmylonites, Khao PLaiKhlong Hin Phao

Khao Phanommylonites

Khanom

6oN-

8oN-

10oN-

12oN-

14oN-

16oN-

18oN-

l98oE

l100oE

l102oE

F CR

KU

CHP NPA

CPA

SO

NK

K

WE

PU

N

CRP

CM

PET

SKN

LP

NB

NS

LY

SBAY

TB

PNSK

KS

KB

LI

EKNW

PR

SPA

KH

HH

Nar

athi

wat

Hig

h

Ko Samui High

MS

MSR MM

NYP

ST

PH

PHR

P

MC

ML

MLA

Cambodia

Malaysia

Myanmar

Laos

Thailand

TH

200 km0 100

WH

WH

PT

AS

GOT

KP

CB

NCB

WH = Western HighlandsNCB = North Central RegionCB = Central BasinKP = Khorat PlateauGOT = Gulf of ThailandPT = Peninsular ThailandAS = Andaman Sea

Uttaradit Fault Zone

Mer

gui R

idge

Cenozoic strike-slip or normal fault

DI = Doi Inthanon

Deformed granite-gneiss that maydisplay Cenozoicmetamorphism andmigmatitesCenozoic folds inKhorat Group

FCR

PU

KU

N

PH

CRP

CM

PETSKN

LP

NB

NSLY

SBAY

TB

PNSK

KS

NWHH

Chiang RaiFangPuaChiang Rai-PayaoNanChiang Mai

PhitsanulokLampang

Sakhon NakhonPhetchabunNong BuaKhorat UdonLad YaoNakhon SawanSuphan BuriAyutthayaKamphaeng SaenThon BuriSakhonPaknamHua HinN. Western

CHPPRKH

NPACPASPA

MASO

NK

KEK

WE

MER

WesternKraEast KraNorth PattaniCentral PattaniSouth Pattani

ChumphonNakhonMerguiSongkhla

KBMerguiKrabi

LI Li

Khmer Prachuap

NMB North Malay BasinSurat Thani Nong y PlongEast Andman

Mae MohMM

MSMSR

NYPEA

ST

PhraeMae SotMae Sariang

PHR

PiPMC

MLMLA

Mae Chaem

TH Thoen Basin

Mae LamaoMae Lai

Cenozoic basin

MER

EA

NMB

Outline ofChao PhrayaBasin

Khao TaphaoKhwam (low-anglenormal fault

ABC

A

B

C

A = simple rifts, B = rifts with post-rift sag basins, (excluding West Siberia)C = passive margins (excluding large deltas)

Total 291 BBOE Total 416 BBOE

A B

Fig. 6

Fig. 7

1.0

2.0

3.0

4.0

5.0

6.0

1.0

2.0

3.0

4.0

5.0

6.0

5 km Approximate depth12 km

22 average dip

A

B

EW

Middle Miocene-Recent

Early Miocene-Oligocene (syn-rift)

Pre-Rift

TWTT

(s.)

Fig. 8

Incipient normal faulting, block rotation,and lower crustal ow

Incipient normal faulting, block rotation,lower crustal ow; fault 1 stop slipping, fault 2 takes over

resulting in passive rotation of fault 1Moho

Moho Moho

Moho

Moho

Extensional basins

Flow of lower crustFlow of lower crust

domino rotationof upper crust blocks

Pre-extension Moho Pre-extension Moho

Pre-extension MohoPre-extension Moho

~ 50 km

Block-rotation model for core complex formation

C1. Converging channel ow of low-viscosity(partially molten) lower crust

C2. Core complex developed at edge of orogenic plateau

Rolling-hinge model for core complex formation

A. WARM CRUST

C. HOTTEST CRUST

B. W HOT CRUST

1 2

1 2 3 4 5Order of faulting

Flowing, partially molten crust

Transfer of thick crusttowards foreland

Channel detachment

PLATEAU: THICK, HOT CRUST FORELAND: COLD CRUST

Upper crust

Lower crust

Fixe

d bo

unda

ry

Solidus

???

? ? ? ??

Doi Inthanon orthogneiss

Paragneiss

Remnant Tertiary basins(e.g. Mae Cham Mine)

Chiang Mai BasinMuscovite Ar/Ar21 1 Ma

Muscovite Ar/Ar18 0.2 Ma

Apatite Fission Track central age 16 Ma 1Maand 14 Ma 1 Ma

5

0

Dep

th (k

m)

CMLANF

Inversion anticline

Triassic granite

Mae Klaeng Granite(Oligocene)

Palaeozoic meta-sedimentary rocks

Mesozoic sedimentary rocks

Cenozoic syn-rift sediments

Doi Inthanon

shape of gneiss dome?

W E

5 km

Lower Detachment

Palaeo-TethysSuture Zone

Meso-Tethys

S-type granitoids

Eastern GranitoidProvince

Western GranitoidProvince

Main-RangeGranitoid Province

I-type S-type

Nan-Sa KaeoBack-arc suture

Loei-PhetchabunVolcanic Belt

SukhothaiArc

Sibumasu Indochina

Early Jurassic

a

A

B

C

W E

v = h0 25 km

-110 -100

-90 -80 -70 -60 Bouger gravity anomaly (mg.)

Orthogneiss Paragneiss

Detachment faultCMLANFDoi Inthanon

Mae ChamBasin

Chiang Mai BasinMae Yuan Fault zone

0

10

20

30

km

Approximate location of Late Oligocene brittle-ductile transition/ Lower Detachent surface (LD)

LD

LD

Mae Tha Fault Payao Fault

Predominantly Palaeozoic rocks, overlying the Lower Detachment (LD)

Moho

Estimated present-day brittle-ductile transition

Fig. 9

Sirikit Field

Western Boundary Fault

UttaraditFault

Phichai Sub-basin

Main normalfault trends

0 10 km

N

A

A

G

Extension mostly during Miocene,Late Miocene-Pliocene inversion

Extension mostly Oligocene-Early Miocene, some mid-late Miocene extension alternating with inversion

Sukhothai Sub-basin

Mixed Miocene andLate Oligocene extensionLate Miocene-Pliocene inversion

Lahan Graben

Nong BuaSub-Basin

Dong ChatSub-Basin

5 km

N

Region of thinsyn-rift sectionand widespreadinversion

Area of welldeveloped riftbasins

Inversion anticlineand syncline

Large normal fault

Invertednormal fault

B

B

Fig. 11

Fig. 13A

Fig. 13B

Fig. 10

? ?

1000

1500

2000

2500

Dep

th (m

etre

s) Spill point?

Oil reservoir

Gas reservoir

Sealing lacustrine shale

200 m

W E

LKU-A05

LKU-A06

LKU-R05

LKU-E17

LKU-E22

LKU-E03

L sands OWC

M sands OWC

P sands OWC

K sands OWC

K sands GWC

TP

TMS

Main Seal

Main Seal K-Sands

UIS

L-sands

P-sands

M-sands

Basal Seal

LIS

Fig. 11

FIg. 12

10 km

?

?

?

?

? ?

?

?

?

?

?

?

?

Small, high-angle fault-bounded depression

Western Boundary Fault becomes active

Uttardit Fault zone becomes active

20-

30 f

ault

zo

ne

dip

s

Wes

tern

Bo

un

dar

y Fa

ult

Hig

h-a

ng

led

no

rmal

falt

s~

45*

dip

s

Late Oligocene Early Miocene, Lower Chum