STYLOLITIC POROSITY IN CARBONATES: A CRITICAL FACTOR FOR DEEP HYDROCARBON PRODUCTION

Petrographic and petrophysical studies were completed on cores from Atokan (Pennsylvanian) carbonate gas reservoirs at depths in excess of 4,000 m in the Chapman Deep field, Delaware Basin, Texas. Porosity concentration is in pure carbonate microfacies of Donezella algal mound, upper inner slope, oolitic shoal and lagoon of a platform depositional model, which are characterized by sutured, medium- to high-amplitude stylolites. The porosity is predominantly stylolitic, enlarged and surrounded by halos of oomoldic, biomoldic, cement-moldic and vuggy types. Non-sutured, low-amplitude stylolites, confined to argillaceous carbonate microfacies, have no influence on porosity development .
The distribution of sutured stylolitization, being microfacies-controlled, allows the use of a depositional model as a prediction tool for locating reservoir microfacies. Under deep-burial conditions, sutured stylolite systems are of critical importance. They permit circulation of undersaturated fluids, become themselves reservoirs together with associated types of secondary porosity, and therefore allow deep commercial gas production.