Abstract

The Three Bar Devonian reservoir is productive from a sequence of highly porous cherts in the Thirtyone Formation. These porous chert deposits, which are widespread in West Texas, have accounted for 650 MM bbl of oil production and contain a similarly large fraction of the remaining hydrocarbon resource. Investigation of the Three Bar reservoir illustrates that reservoir heterogeneity is a result of the complex overprinting of original depositional patterns by chert diagenesis, karsting, and tectonics.

Chert porosity development is the result of the formation of spicule molds and intercrystalline pores and micropores during differential silica diagenesis. In chert proximal to limestone beds, diagenesis has resulted in porosity occlusion. Diagenesis of siliceous sediments more distal to limestone beds was porosity retentative. Styles of chert brecciation indicate that chert diagenesis proceeded at slower rates in porous cherts, suggesting that chert porosity is a function of the rate of silica diagenesis, which in turn may be controlled by interaction with carbonate. Porosity heterogeneity in chert is thus a function of the original and early post-depositional distribution of carbonate sediment and its apparent impact on rates and products of chert diagenesis. Relatively little porosity in chert is due to leaching of carbonate.

Karsting of the reservoir during major periods of uplift created porosity zones in limestone and locally reduced vertical heterogeneity in chert zones but did not substantially affect chert porosity trends. Faulting associated with Pennsylvanian uplift of the sequence resulted in creation of abundant fractures. These fractures serve as preferred flow pathways for fluid movement and make effective drainage of matrix pores difficult.

Consideration of the multiple causes of heterogeneity and their effects suggests that targeted infill drilling will result in improved recovery in the Three Bar field. Comparison of the Thirtyone Formation reservoir sequence in the Three Bar field with other chert reservoirs in West Texas indicates that the styles of heterogeneity documented here are widespread and that this reservoir may serve as a model for exploitation of many reservoirs of this type.