burial history modeling and pressure prediction in deep water offshore trinidad 2003

8/2/2019 Burial History Modeling and Pressure Prediction in Deep Water Offshore Trinidad 2003

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Copyright 2003, Offshore Technology Conference

This paper was prepared for presentation at the 2003 Offshore Technology Conference held inHouston, Texas, U.S.A., 58 May 2003.

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AbstractPressure prediction in deepwater Trinidad is a key component

of well planning and prospect risking. Similar to thedeepwater G.O.M., shallow water and gas flows are common,

and many wells have been terminated due to narrow drilling

safety margins both on the shelf and in deepwater. Extremely

high sedimentation rates, ranging between 1 and 3 km/Myr

since the Pliocene, and high relief structures (>1.5km) suggest

that the primary pressure mechanism in the basin is

compaction disequilibrium compounded by lateral transfer.

Geologic information suggests that burial history modelingshould be an effective means of pressure prediction in the

basin. By calibrating specific model scenarios (both 3D and

1D) to reliable geologic data and observations (pressure data,

density logs, expulsion feature distributions, seismic velocity,temperature, and gravity data), burial history models can

adequately predict subsurface pressure and temperature in the

area. In addition, in areas with little or no local well data, a

burial history model may be calibrated to remote observations

of gravity and seismic properties, which allows one to develop

specific permeability structure scenarios. These alternate

scenarios may be risked with appropriate scenario probabilityweighting.

A key variable in the models is the presence or absence of a

thick, low permeability pelagic shale section near the Plio-

Pleistocene boundary. This claystone seal contrasts with theoverlying Pleistocene section composed mainly of silty shaleand siltstone. Gamma ray logs do not clearly show the

lithologic transition and the facies change is not evident from

seismic data, although it can be discerned from mud-log

descriptions, paleontologic, and surface area data. Inclusion

of this interval with the appropriate composition in pre-drill

models adequately predicted the pressure gradients observed

during drilling, while models assuming compositions moresimilar to overlying intervals did not.

Results of pre and post-drill burial history modeling show

that such models can be used for quantitative pressure

prediction in environments of rapid clastic deposition such as

Trinidad. Pre-drill predictions fell within 0.5 ppg EMW of

observed pressures at 4 wildcat wells drilled in the area. The

models are particularly useful in scenario analysis ofpermeability structure and large-scale fluid flow when cross

referenced with physical property data.

IntroductionDeepwater offshore Trinidad is an area of rapid clastic

sedimentation from the Pliocene to present and lies just Southof the Barbados accretionary complex (Fig. 1). Drilling on the

shelf has frequently encountered significant overpressure (1)

It was therefore anticipated that the deepwater area would

have high risk for overpressure, and that a full pore pressure

and fluid flow evaluation effort was warranted to support theexploration effort in the area.

Ideally, evaluation of fracture and pore pressures can be

constrained by many types of data. Despite that fact, the

analysis is often restricted to offset well or seismic velocity

data. In order to reduce as much as possible the pressure-

related uncertainties associated with exploration in a frontier

area, an integrated approach to pressure prediction was

implemented for exploration in offshore Trinidad.The process for evaluating the pressure environmen

proceeded from regional to prospect scales and combined

inputs from seismic, offset wells, gravity surveys and burial

history models in one to three dimensions. An emphasis wasplaced on understanding the controls on fluid flow and stresses

in the system and their impact on pore and fracture pressures

This allowed for better sensitivity analysis and scenario

development during the technical work. Well results were

used to fine tune the models and decide upon the most

appropriate ongoing approach.

Primary Causes of Overpressure

Rapid loading.Sedimentation rates in offshore Trinidad arehigh, from 1-3km/Ma (see Figs. 1 and 2), and appear to be themain cause of overpressure in the area under considerationThe sedimentation rates are comparable to rates of deposition

observed in other overpressured clastic basins, such as the

Gulf of Mexico and the South Caspian.

Lithology as a control on overpressure distributionPatterns in Figures 3 and 4 indicate that at the offset wells

(A,B,C,D,E) the onset of severe (near lithostatic) pressure isrelated to a transition from a silt and sand-prone environmen

to a bathyal, claystone dominated section (Figures 3 -6). This

section is an effective seal isolating the deeper section from

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Burial history modeling and pressure prediction in deep water offshore TrinidadT.G. Fitts, ExxonMobil Upstream Research, M. Cheng, and M. Quinn, ExxonMobil Exploration


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the shallow section on a large (>10km) scale. The lithologic

and permeability change with depth can be demonstrated bythe shift in surface area shown in Fig. 6. Faults and other

conduits are required to connect the two sections on short

(5 km) are likely to be smal

because the rocks involved are low porosity compared to

overlying sediments. The hypothesis that most of the fluid

and solids actually expelled from mud volcanoes originates inhigher porosity shallower strata is supported by age dating of

solid materials.

Pressure at depths of 4-6 km may be regulated by episodic

mud volcanism at deep structural highs, but it is unclear to

what extent this mechanism helps regulate pressures in the

shallower section. Pressures approach lithostatic in much of

this section, so that it does not appear that the mud volcanoes

provide an efficient drainage conduit for the shallow sectionAs described, fluid sources related to diagenesis and source

maturation probably are limited in their impact on the shallow

section pressures because the pressure relief valves (mud

volcanoes) reach the surface and water volumes largely bypassthe intervening section.

The deepwater Trinidad system appears to have little bulk

water flow between the three levels of deep source

intermediate overpressure and shallow reservoir. This need no

be the case for buoyancy-driven hydrocarbon charge. The

fracture networks associated with mud volcanoes and

structural trends may be conduits for hydrocarbon charge orleakage without strongly impacting water flow in the system.

Potential external fluid source: subduction zone fluids. An

additional potential source of overpressure is metamorphicfluid transported laterally from the subduction zone to the

E/NE of the ExxonMobil blocks. Fluids originating in the

subduction zone flow along localized conduits in the

detachment zone separating relatively undeformed sediments

from highly deformed overthrust materials in the Barbados

accretionary wedge (see Figure 14, (8), and the extensive ODP

documentation of Legs 110 and 156). Chemical andgeophysical signatures suggest that the lateral migration

occurs in a localized fashion in the detachment zone and along

thrust faults which pervade the accretionary wedge section (6


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9, and others). Near lithostatic pressures are probably required

for volumetrically significant, rapid migration along the near-horizontal detachment (10,11).

Within the areas of interest for this study, it is expected

that fluid flow sourced from the subduction zone would be

deflected parallel to the subduction zone by the large sediment

load of the Columbus basin (e.g. see Figure 14 (8)). The

Columbus basin supra-detachment section is an order ofmagnitude thicker than the supra-detachment section in the

Barbados accretionary prism where flow has been best

documented. Vertical migration of these fluids may also bemore limited than it is to the North because the overlying

section is not pervasively deformed as it is in Barbados, as

well as being much thicker.Because the fluid budget due to compaction of the

sediment thick is already so large, the relative volume of the

subduction zone related fluids and their relative contribution

to overpressure is predicted to be small in the Columbus basin.

Unlike work done in the accretionary prism proper (12,13)

was not possible to use seismic velocities or impedance at

detachment level to evaluate where low stress segments of the

faults might be due to the great depth of the detachment.

Hence it was initially proposed to test the idea that forwardmodeling of pressures may not require accounting for their

presence in a predictive mode.

Seismic interval velocities as a constraint onregional pressure modelingMethodology. The lateral and vertical distribution and

variation in contractor-picked seismic interval velocities)were

used to evaluate patterns of overpressure in the EM 1 and 2

blocks. A map of the top overpressure pick from the contractor

picked velocities is shown in Figure 9. The map is used as a

qualitative descriptor of the regional presence of overpressure

and general depth trend only.Contractor picks were of acceptable quality for delineating

regional trends but are picked in order to improve imaging

rather than to determine accurate earth velocities. Because lessresolution is typically needed for imaging than for pressure

prediction additional velocity work is usually required in order

to predict the magnitude of pressure. In addition, physically

unrealistic stacking velocities sometimes produce adequate

images due to acquisition parameters and structural

complexity, so screening is necessary.

For site specific work interval velocities were re-picked

using the gather moveout to improve interval velocityaccuracy for well planning. Regional trends and patterns

appeared to be stable overall. Time-depth conversion was alsoperformed using these functions calibrated to offset wells. At

specific locations velocities were transformed to effectivestress using a modified version of the Bowers method (14).

Significance of pattern for regional pore pressure model.

The lateral and vertical distribution and variation in interval

velocities imply a ubiquitous zone of severe overpressure

beginning at a slightly variable stratigraphic depth within the

Late Pliocene. The magnitude and thickness of the velocityreversal appears to be most pronounced on structural crests

and is sometimes absent in synclines (Fig. 9). The pattern

supports the hypothesis of lateral fluid migration towards

highs beneath a regional/sub regional seal near the top of the

Pliocene section.

Gravity modeling: Implications for pressure modelsMethodology. Based on these results 3-D forward modeling

of high quality, marine gravity data was done to provide a

quick (


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predictions. The results from 3D models and evidence for

lateral transfer were used to support the use of a centroidmodel with the 1D results as a final solution. The one-

dimensional model was most accurate in the shale prone parts

of the section, as lateral flow is less important in these

intervals.

Methodology. The pore pressures and rock properties in theWells, A, B, C, D and E were forward modeled using

relatively simple burial history models with lithology derived

from gamma ray and/or resisitivity logs and age-depthinformation from paleontology and seismic interpretation.

A velocity/effective stress calibration data set was used to

produce sonic velocity profiles in the models. Sand and shalemechanical properties were calibrated to match observed

properties at Well C. Results of simple models for Wells A

and B are shown in Figure 4. The main difference between the

wells is the amount of sand in the section. The greater

abundance of sand at Well A leads to lower pressures in the

shallow section, although pressures in the deep section below

the main seal interval are similar. The equivalent shallow

section is sufficiently sandy at the shelfal Wells D and E that

the section is normally pressured above the regional seal.Shelfal Well C has a shaley shallow section despite its less

distal position spatially, and has a shallow onset of

overpressure.The simple 1-D model results are consistent with the

available pressure and velocity data in the shallow sections,

although model resolution is coarser than log data. The fits at

Wells A, B and C, which have sands in the shale-prone over-

pressured interval, are improved by incorporation of a pressure

source due to flow from down-dip (Figure 4).

Incorporation of lateral flow effects. The risk of mechanical

seal failure due to pore pressure exceeding the fracturegradient was evaluated as part of the pore pressure exercise.

Results were used as input to risking exercises. Where

projection of minimum pore pressure estimates from thecentroid depth exceeded crestal fracture gradient values even

for a water gradient, the interval was considered to have

extremely high risk for mechanical seal (probable column

height = 0). Figure 4 illustrates such a case. This occurs most

notably in the deep overpressured sectionHydrocarbon column heights using the centroid model are

different than that those that would be predicted using the

estimate of 1-D shale pore pressure at the well location,because shale pressures have a higher gradient than do sand

pressures. A good discussion of ways to model these effectscan be found in (15). The effect is most pronounced insections having shale pressures significantly in excess of

hydrostatic and having sand beds with high relief. For beds

with total vertical relief < 400m the effect is usually below the

resolution of pressure prediction methods. It is important to

use the appropriate procedure to estimate reservoir pressures

on beds having high relief.

Uncertainty estimation. The range on the 1-dimensionalgeologic models is largely produced from estimated

uncertainty in lithology prediction in the shallow section. In

particular, the shale composition and presence or absence of

discrete sand beds strongly controlled the shape and

magnitude of pressure profiles. Variations in the age model

within the uncertainty range produced relatively smalperturbations in comparison, except in the shallow section

burial history modeling and pressure prediction in deep water offshore trinidad 2003

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