Petrophysical Characteristics, Depositional Systems, and Model of Geological Evolution in the Golden Lane Carbonate Sequences

ABSTRACT The Golden Lane area located in the southern Gulf of Mexico is a famous rimmed carbonate platform with significant oil production up to date. Different geological models have been developed to account for the formation of hydrocarbon fields in this area. With accumulation of significant data over the last decades, it came to a time to re-evaluate the mechanisms that controlled the accumulation of reservoir rocks and formation of hydrocarbon fields. In this study we conducted an integrated analysis on the depositional environments, sedimentary facies, sequences variations and geological evolution of the Golden Lane carbonate system in order to provide guidelines for future exploration and field development. We have reviewed a large amount of carbonate cores from various area in the Golden Lane, and compared the core data to petrophysical logs as well as seismic profiles. Our study mainly concerns the Cretaceous interval that contains the major oil producers -- the El Abra and Tamabra Formations. Two major mechanisms have been proposed to explain the geological evolution of the study area. First, the pre-Mesozoic basement experienced a period of stress relaxation, accompanied by post-mountain-building uplift. The Golden area may have started from a shallow marine carbonate bank, and this shallow sea carbonate bank is similar to the Bahama platforms today [Matthews, 1984; Enos, 1983] which rise abruptly out of several hundred meters of water and are constructed by the accumulation of carbonate and evaporite sediments at or near sea level during the continuing subsidence of this area since Cretaceous time. Second, overall sea level was continuously rising during the entire Cretaceous. The magnitude of this sea level rising was in the order of 300 to 400 meters based on a calibration from seismic stratigraphic records (Vail, 1977; Pitman, 1978). Our model differs from previous studies (Coogan, 1972; Minero, 1991) in the major mechanisms controlling the formation and distribution of the thick carbonate sequences, and their relationship to petroleum accumulation. By the end of the Cretaceous, sea-level rise reached its peak. The upper Cretaceous basin facies carbonate covers the entire area. The Tertiary marked another episode of strong tectonic uplift, accompanied by a large-scale sea level falling. A layer of bentonite was deposited on top of the El Abra Formation, marking the end of Mesozoic marine deposit in the area. The area then underwent a period of depositional hiatus, which presumably caused a significant subaerial exposure on top of the Abra Formation. This suggests an important mechanism for the formation of hydrocarbon reservoirs in the study area. Another mechanism for the formation of oil reservoirs is through the development of natural fracture system. Such a general geological model allows us to make certain predictions and compare these predictions to the oil fields over the entire Golden Lane in consecutive studies.