hydrocarbon production and tepetate fault zone, la

GCAGS/GCSSEPM Transactions � Volume 53 � 2003548

HYDROCARBON PRODUCTION AND SURFACEEXPRESSION OF THE CHINA SEGMENT OF THE

TEPETATE FAULT ZONE, LOUISIANA

Byron Miller1 and Paul V. Heinrich2

1Louisiana Geological Survey, 2079 Energy, Coast, and Environment Building,Louisiana State University, Baton Rouge, LA 70803; E-mail: [email protected]

2Louisiana Geological Survey, 2069 Energy, Coast, and Environment Building,Louisiana State University, Baton Rouge, LA 70803; E-mail: [email protected]

ABSTRACT

The China Segment is a portion of the Tepetate Fault Zone that traverses St.Landry, Acadia, and

Jefferson Davis parishes. It is part of the larger Baton Rouge-Tepetate Fault System that is a major

down-to-the-basin growth fault zone that traverses the northern portion of south Louisiana. This

fault system exhibits syndepositional growth in the late Eocene and Oligocene periods with surface

fault-line scarps and floodplain deformation indicating movement during Pleistocene and Holocene.

The Baton Rouge Fault Zone comprises the eastern and central portion of the fault system, the

Tepetate Fault Zone comprises the western portion and presents a more complex, sinuous, and

segmented trace in the subsurface.

The China Segment exhibits surface expression consisting of a series of scarps that comprise the

China fault-line scarp. The fault-line scarps exhibit displacement ranging from 1.5 to over 3 meters.

These scarps offset the surface of the Prairie Alloformation and associated abandoned river channels

indicating reactivation and movement during the Pleistocene. Deformation of flood plains, where

crossed by the trace of Tepetate Fault Zone, demonstrates Holocene fault movement.

Eight named hydrocarbon producing fields associated with the China Segment have cumulative

production of 106 MMBO + 410 BCFG. Producing structures are predominately fault generated

rollover anticlines. The China Segment is also recognized as a barrier to freshwater flow in regional

freshwater aquifers. The affect of fault reactivation on hydrocarbon entrapment and freshwater

resources is uncertain.

INTRODUCTIONThe Baton Rouge-Tepetate Fault System is a major east-west trending, down-to-the-basin

growth fault zone that traverses the northern portion of south Louisiana. The Baton Rouge Fault

54953rd Annual Convention � Baton Rouge, Louisiana

Zone comprises the eastern and central portion of this fault system; the Tepetate Fault Zone com-prises the western portion of the fault system.

This fault system exhibits syndepositional growth in the late Eocene and Oligocene periods,evidenced by expanded stratigraphic intervals downthrown to the fault. Pronounced surfaceexpression (particularly along the Baton Rouge Fault Zone), surface fault-line scarps and floodplaindeformation are indicative of a renewed period of fault movement during the Pleistocene andHolocene.

Many hydrocarbon producing fields are located along the Baton Rouge-Tepetate Fault System.Most of the fields are rollover anticline structures associated with the fault system, however, somefault truncation traps are present.

This paper focuses on the China Segment of the Tepetate Fault Zone. The China Segment is aportion of the Tepetate Fault Zone that traverses St.Landry, Acadia, and Jefferson Davis parishes.

SURFACE GEOMORPHOLOGY

Within Southeast Louisiana, the surface expression of the Tepetate fault zone consists of a seriesof low fault-line scarps (Heinrich 1997; Heinrich 2000). Along the China Segment of the Tepetatefault zone, its surface expression consists of a series of fault-line scarp segments that extend fromabout one mile (1.6 km) east-southeast of Fontenot, Louisiana, within Jefferson Davis Parish toabout 2 miles (3.2 km) north of Tepetate, Louisiana, in Acadia Parish. These fault-line scarps arenamed the China fault-line scarp after the China Cemetery, which lies on the crest of this scarp inSec. 19, T. 7 S., R. 3 W. about 3.5 miles (5.6 km) south of Elton, Louisiana (Fig. 1).

The China fault-line scarp offsets the terrace surface of the Prairie Allogroup along its entirelength. It consists of low, gentle scarps with a relief of about 7 to 12 ft (2 to 3.6 m). This scarp occursas a noticeable break in the gently sloping surface of the Prairie Allogroup that is apparent on 7.5minute topographic maps (Figs. 1, 2, and 3). In places, the China fault-line scarp offsets relict RedRiver courses. One excellent example of a relict Red River course offset by the China fault-line scarplies about 3.6 miles (5.8 km) south-southeast of Basile, Louisiana (Fig. 2).

Between Fontenot, Louisiana, and the Calcasieu River, the Tepetate Fault Zone lacks surfaceevidence of active faulting. However, about 7 miles (11 km) west of the western end of the Chinafault-line scarp, Paine (1962) documented fault displacement within fluvial sediments of the PrairieAllogroup. The faulting within these fluvial sediments, which dies out before reaching the surface,is exposed in the sides of Wolfe gravel pits within Sec. 29, T.7 S., R. 6 W. This exposure lies about4000 ft (1220 m) north of the Hawkins and Cummings, King Corp. No. 2 well, which cuts the faultzone at a depth of 6477 ft (1974 m). Thus, the dip required for this fault to be connected with the

Figure 1. Excerpt from the Elton 7.5' topographic quadrangle, Jefferson Davis Parish. It presents the Chinafault-line scarp, surface trace of fault (solid line), subsurface trace of faults at approximately -6600 feet(dashed line), relict Red River course (gray line marked "ac"), and China Cemetery (c).

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fault zone is about 58°, which is reasonable for a near-surface Gulf Coast listric growth fault(Heinrich, 2000).

Holocene movement of the China Segment of Tepetate fault zone has affected the floodplain ofBayou Serpent in Jefferson Parish, about 4.7 miles (7.5 km) southeast of Kinder, Louisiana (Fig 3).Here, a linear east-west trending swamp lies perpendicular to the valley of Bayou Serpent. Thislinear swamp is located just south of the projected trace of the China fault segment as indicated byadjacent segments of the China fault-line scarp. This linear swamp appears to be a sag pond devel-oped within the Holocene floodplain of Bayou Serpent. In addition, ponded channel segments ofBayou Serpent that gradually disappear downstream abruptly end at this linear swamp segment

Figure 2. Excerpt from the Basile 7.5' topographic quadrangle, Acadia Parish. It presents the China fault-line scarp, surface trace of fault (solid line), and relict Red River course (gray line marked "ac").

Figure 3. Excerpt from the Kinder 7.5' topographic quadrangle, Allen Parish. It presents the China fault-linescarp, surface trace of fault (solid line), subsurface trace of faults at approximately -6600 feet (dashed line),relict Red River course (gray line marked "ac"), sag pond (sp?), and ponded channels (pc).

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(Fig. 3). This indicates that this segment of Bayou Serpent is downwarped relative to its floodplaindownstream. The abrupt truncation of the ponded channels at the east-west trending, linear swampis a strong indication that the floodplain on the south, and downthrown, side of the fault trace hassubsided relative to the floodplain north, and upthrown, to the fault trace.

HISTORY OF FAULT MOVEMENT

An examination of the subsurface data from the Tepetate fault zone within the China Segmentfound insufficient data to reconstruct a complete history of its movement. However, Heinrich (2000)was able to reconstruct a relatively detailed history of movement along the segment of the Tepetatefault zone associated with the De Quincy fault-line scarp. The data for this reconstruction camefrom a combination of geophysical logs for salt water disposal, oil, gas, and water wells andGidding Soil Probe borings drilled on either side of fault-line scarp.

The initial movement of the Tepetate fault zone occurred during the deposition of the YeguaGroup. This movement consisted of normal growth faulting associated with the deposition of thickdeltaic and shelf sediments. During the Oligocene, Miocene and most of the Pliocene, the fault zoneremained inactive.

As illustrated by Heinrich (2000), a second period of movement is evident along the Tepetatefault zone (Fig. 4). The decrease of vertical offset with depth down to a depth of 700 ft (200 m)indicates that the second period of movement along the Tepetate fault zone started sometimeduring the late Pliocene Epoch and continued throughout the Pleistocene Epoch. As previouslydiscussed for the floodplain of Bayou Serpent, Holocene movement has occurred along segments ofthe Tepetate fault zone. Heinrich (2000) noted similar evidence of Holocene movement along the DeQuincy fault-line scarp in Calcasieu Parish. This history suggests that the reactivation of theTepetate fault zone is related to the rapid loading of the Louisiana continental shelf by sedimentstransported down the Mississippi River since the beginning of continental glaciation during the latePliocene, as hypothesized by Nunn (1985), and continues into the present.

HYDROCARBON PRODUCTIONThe Baton Rouge-Tepetate Fault System has been a significant hydrocarbon producing feature

with a large number of hydrocarbon producing fields associated with this fault system. Eightnamed hydrocarbon producing fields are associated with the China Segment of the Tepetate FaultZone. These named fields (West Edna, Edna, China, South Elton, West Tepetate, Tepetate, Richie,and South Bayou Mallet) have cumulative production of 106 MMBO + 410 BCFG. Several fields arestill producing although most of the fields are nearing depletion.

Structure maps for the producing fields at approximately equal depths were gathered frompublished and public sources and plotted to show their general structural shape and orientationrelative to the surface trace of the China Segment of the Tepetate Fault Zone (Fig. 5). The mapsshow that the producing structures associated with the China Segment are predominately anti-clines. The anticlines are presumed to be fault generated rollover anticlines based on their proxim-ity to the Tepetate fault and elongated structural closure oriented sub-parallel to the Tepetate faulttrace.

The structural shape maps show that few fields associated with the China Segment of theTepetate fault have fault truncation seals as a trapping mechanism for the hydrocarbons. A reviewof the structure maps for each unitized reservoir in each field, available as part of the public recordfrom the Louisiana Office of Conservation, confirms that few of the reservoirs show fault truncationwith the China Segment of the Tepetate fault as a hydrocarbon trapping mechanism. This is ingeneral agreement with observations from the Baton Rouge fault where a prior study revealed fewof the associated hydrocarbon pools were the result of fault truncation trapping mechanisms (Milleret al., 2002). This is most likely due to the Baton Rouge-Tepetate Fault being permeable to fluidflow, an assumption supported by groundwater studies.

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The Baton Rouge fault is demonstrated to be a barrier to groundwater flow on a regional scale,separating freshwater aquifers north of the fault from saltwater aquifers south of the fault (Miller etal., 2002). However, several groundwater studies have researched saltwater intrusion into freshwa-ter aquifers immediately adjacent to the Baton Rouge fault and contend the Baton Rouge fault ispermeable to groundwater flow. These studies generally agree the saltwater intrusion is the resultof saline water being drawn across the fault due to high pumpage rates, from saltwater aquifersjuxtaposed to freshwater aquifers. However, Kuecher (1997) suggested the Baton Rouge fault wasacting as a vertical conduit for saltwater flow from deeper saline aquifers into shallower freshwateraquifers. Regardless of the mechanism involved, the observations support the assertion that theBaton Rouge fault is not a sealing fault, but allows localized fluid leakage across the fault.

Like the Baton Rouge Fault, the Tepetate Fault is recognized as an impediment to regionalgroundwater flow. Regional groundwater cross sections show a major change in the depth tofreshwater across the fault, with the base of freshwater exceeding 2000' in depth north of the fault toless than 500' in depth south of the fault (Smoot, 1989). This is very similar to that observed for thegroundwater aquifers around the Baton Rouge Fault. In addition, the small number of fault trunca-tion hydrocarbon traps suggests the Tepetate Fault may exhibit similar hydrocarbon trappingproperties, allowing localized fluid leakage across the fault.

Possible localized fluid leakage across the China Segment of the Tepetate fault is supportedby the presence of hydrocarbon-barren structures adjacent to the fault. A review of the field struc-tural shape maps reveals several barren structures adjacent to the China Segment of the Tepetatefault, some of which are in superior structural position to the adjacent hydrocarbon pools (Fig. 5).

Figure 4. Plot of separation versus depth for fault associated with the De Quincy fault-line scarp in CalcasieuParish. "Sb15" and "Sc1" are sandstone beds; "Shd1" is shale bed; and "N sand" is Nodosaria Sand inCross-Section A of Heinrich (2000). (Reprinted by permission, from Figure 6, Heinrich 2003. Copyrighted bythe Louisiana Geological Survey.)

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For example, China field shows a barren structural nose feature adjacent to the China Segment ofthe Tepetate fault and syncline separated from established production. Richie field displays arollover anticline and a structural nose feature with hydrocarbon pools stratigraphically trapped onthe east flank of the structures; neither of the hydrocarbon pools are depicted as having any signifi-cant contact with the China Segment of the Tepetate fault. West Edna and South Bayou Malletfields both have barren fault wedge traps adjacent to the China Segment of the Tepetate fault.

The reactivation of the Baton Rouge-Tepetate Fault System during the Pleistocene andHolocene may have impacted the hydrocarbon charged reservoirs and affected the sealing proper-ties of the fault. However, the timing of hydrocarbon migration, the charging of the reservoirs withhydrocarbons, and the effects of fault reactivation, are not known.

CONCLUSIONSThe China Segment of the Tepetate Fault Zone exhibits syndepositional growth during the late

Eocene and Oligocene periods, with a second period of movement apparent during Pleistocene andHolocene. Surface fault-line scarps and floodplain deformation are evidence of fault reactivationand movement.

Hydrocarbon producing structures associated with the China Segment of the Tepetate FaultZone are predominately fault generated rollover anticlines with eight named hydrocarbon produc-ing fields posting cumulative production of 106 MMBO + 410 BCFG. The effect of fault reactivationon hydrocarbon accumulation and entrapment is not known.

ACKNOWLEDGMENTS

I would like to thank my co-author, Paul Heinrich, whose idea and initiative spearheaded thisinvestigation.

REFERENCESHeinrich, P. V., 1997, Pleistocene fault-line scarps and neotectonics in Southwest Louisiana: Geological Society of

America Abstracts with Programs. v. 29, no. 3, p. 23.Heinrich, P. V., 2000, The De Qunicy fault-line scarp, Beauregard and Calcasieu parishes, Louisiana: Basin Re-

search Institute Bulletin. v. 9, p. 38-50.

Figure 5. Structural shape maps of hydrocarbon fields associated with the China Segment of the TepetateFault Zone. Reservoir extent shown by shading. Surface trace of the fault (A); fields associated with thefault: 1. West Edna; 2. Edna; 3. China; 4. South Elton; 5. West Tepetate; 6. Tepetate; 7. Richie; and 8. SouthBayou Mallet.

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Kuecher, G., 1997, New transport mechanism found for salt water intrusion: Argonne National Laboratory, http://www.es.anl.gov/htmls/transport.html.

Miller, B., McCulloh, R.P., John, C.J., Harder, B., Bourgeois, R., 2002, Occurrence and structural control of hydro-carbon production associated with the Baton Rouge Fault Zone, Louisiana: AAPG Annual Meeting Abstractswith Program, p. A123 .

Nunn, J.A., 1985, State of stress in the northern Gulf Coast: Geology. v. 13, p. 429–432.Smoot, C.W., 1989, Geohydrologic Sections of Louisiana: U.S. Geological Survey Water Resources Investigations

Report 87-4288.Paine, W. R., 1962, Geology of Acadia and Jefferson Davis parishes: Geological Bulletin no. 36. Louisiana Geologi-

cal Survey, Baton Rouge, 277 p.

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