Burial History Modeling and Pressure Prediction in Deep Water Offshore Trinidad

Abstract

Abstract Pressure prediction in deepwater Trinidad is a key component of well planning and prospect risking. Similar to the deepwater 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 modeling should 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 probability weighting. 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 the overlying Pleistocene section composed mainly of silty shale and 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 more similar 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 of permeability structure and large-scale fluid flow when crossreferenced with physical property data. Introduction Deepwater offshore Trinidad is an area of rapid clastic sedimentation from the Pliocene to present and lies just South of 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 the exploration 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 pressurerelated 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 environment proceeded from regional to prospect scales and combined inputs from seismic, offset wells, gravity surveys and burial history models in one to three dimensions.