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.
This paper was selected for presentation by an OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, as
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abstract must contain conspicuous acknowledgment of where and by whom the paper waspresented.
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