Paleoenvironments, diagenesis, and sequence stratigraphy of the mid-cretaceous section (albian/cenomanian) in the Main Pass Field area, northeastern Gulf of Mexico

Abstract

The Main Pass Block 253 Field, northeastern Gulf of Mexico, produces oil from mid-Cretaceous carbonate reservoirs at 8,700 to 8,900 feet. The field is located on Main Pass Blocks 253 and 254 and is situated along the present-day continental shelf margin, which closely approximates the shelfedge during Early Cretaceous time. This field is significant because it demonstrates the oil potential of Lower Cretaceous carbonates in the northeastern Gulf of Mexico. Core and thin-section studies of the Washita reservoir rocks, Late Albian to Early Cenomanian in age, show that these carbonates accumulated as part of a reef-shoal complex in a carbonate rimmed-shelf system. A general progradational trend in the section is evident: shelf-margin reef facies are overlain by back-reef and patch-reef facies represented by middle-to-inner-shelf carbonate deposits. Overall, 10 distinct cycles grading upward from mud-supported bafflestone, wackestone, and packstone to grain-supported rudstone and grainstone are observed in core. These coarsening-upward cycles reflect an increase in energy upsection. The boundaries of the cycles are defined by physical surfaces of erosion or non-deposition, or by abrupt changes in facies. The major constituents in the reef facies are rudists, including caprinids and radiolitids. These organisms are in life position in the lower part of the sequence. A major unconformity is observed in the core. Washita carbonates are overlain by Campanian chalk. The chalk was dated using nanofossil associations. This unconformity is the regional mid-Cretaceous unconformity of the northeastern Gulf of Mexico as recognized on seismic profiles. This unconformity probably contributed to the intense diagenetic alteration of the Washita carbonates observed in core. Diagenetic processes observed in the core include dolomitization, neomorphism, pressure solution, and karstification.