Distribution of overpressure and hydrocarbon phases in the Eugene Island Block 330 Field, Gulf of Mexico, support previous suggestions that episodic slip on critically stressed faults provides dynamic control of hydrocarbon column heights in some of the fault blocks. On the hanging-wall of the field's growth fault system, pore pressures in the OI reservoir at the top of the structure are noticeably higher than porosity-based pressure predictions for the top seal. In addition, water phase pressures in the reservoir vary markedly from fault block to fault block, implying significant differences in reservoir plumbing. Fault blocks with relatively lower water phase pressures contain large gas columns with small oil rims. Down-flank spill or leak points control the total column height, while capillary seal capacity limits the amount of gas. In this situation, gas leaks at the crest of the structure and is unable to flush oil out of the trap. In contrast, fault blocks with markedly higher water phase pressures are significantly under-filled, and contain small oil columns. As the pressure at the top of the columns are within =92% of the least principal stress, we propose that critically stressed faults control trap fill. In this scenario, any additional hydrocarbon charge increases pressure, inducing slip on bounding faults and causing gas leakage from the trap leaving an oil column behind.
1. INTRODUCTION
Post-drill analysis of hydrocarbon fields world-wide indicates that controls on trap fill can be classified broadly into two main categories: geologic controls and dynamic controls. Under hydrostatic conditions, capillary entry pressure is the dominant geologic control on hydrocarbon migration and entrapment in the subsurface [1]. Sales [2] showed that capillary entry pressure can control not only the total hydrocarbon column height a trap holds, but also oil and gas column heights. Similarly, this paper shows that dynamic mechanisms that control total trap fill (hydraulic fracturing and dynamic fault capacity) can also control oil and gas distribution in hydrocarbon bearing reservoirs.
2. DYNAMICALLY CONSTRAINED HYDROCARBONS
Areas undergoing rapid sedimentation like the Gulf of Mexico are often characterized by a normal faulting environment where the overburden is the maximum principal stress (i.e., Shmin =?SHmax =?Sv). In such areas the pore pressures in compacting shales are generally expected to be higher than in adjacent sands units because of their low permeability and relatively poor drainage during compaction. However, there have also been models published predicting the contrary (i.e., pore pressures in sands are higher than in adjacent shales) under appropriate circumstances. The centroid as presented by Traugott and Heppard [3] is such a model. Finkbeiner et al. [4] proposed a modified version of this model that uses pore pressures values at the top of reservoir sands and integrates this pressure information with the ambient in situ state of stress in the shales to evaluate dynamic mechanisms for fluid migration and accumulation. These mechanisms enable us to establish bounds for the maximum column heights supported by the fluids trapped in the reservoir since it is the sealing capacity of the overlaying top seal or the fault against which the reservoir abuts that controls a critical pore pressure in the underlying sand.
