Abstract

Upper Jurassic (Oxfordian) thrombolite boundstone and doloboundstone are proven hydrocarbon reef reservoirs in the onshore northeastern Gulf of Mexico. These Oxfordian thrombolite buildups attained a thickness of 190 ft (58 m) and are as much as 2.4 mi² (6.2 km²) in area. They developed in a shallow water setting in less than 30 ft (10 m) of water. Thrombolite buildups also occur in Lower Cretaceous (Berriasian to Barremian) strata in the onshore central Gulf of Mexico. A representative thrombolite is observed in the well log signatures and core samples from the Lawrence L. McAlpin #1 well, Vernon Parish, Louisiana, which attains a thickness of 35 ft (11 m). Seismic data show that this thrombolite buildup developed in a fore-reef slope setting in up to 300 ft (90 m) of water on the upper part of the continental slope. The thrombolite boundstone has a micritic fabric and is interbedded with fossiliferous wackestone. Although this thrombolite boundstone facies could have high reservoir potential where dolomitized, the geographic distribution of this facies has not been delineated. These thrombolites were formed by eurytopic organisms, which were not restricted by water depth, salinity, temperature, or light penetration. Their origin and growth typically corresponded to times of rising sea level under low background sedimentation rates and low-energy conditions. The demise of the thrombolites resulted from the development of stenotopic, higher-energy paleoenvironmental conditions that supported metazoan communities. These conditions are associated with times of reduction in the rate of sea-level rise.