Impact of regional variation in detrital mineral composition on reservoir quality in deep to ultradeep lower Miocene sandstones, western Gulf of Mexico

摘要

Future exploration in lower Miocene sandstones in the Gulf of Mexico will focus increasingly at depths greater than 4.5 km, and reservoir quality will be a critical risk factor in these deep to ultradeep reservoirs. The goal of this study was to understand the variation in reservoir quality of lower Miocene sandstones across the western Gulf of Mexico. To do this, we examined regional variation in detrital mineral composition, diagenesis, and reservoir quality in five areas: (1) Louisiana, (2) upper Texas coast, (3) lower Texas coast, (4) Burgos Basin, Mexico, and (5) Veracruz Basin, Mexico using petrographic and petrophysical data from depths of 0.9-7.2 km.There are strong variations in mineraiogical composition within the study area. Lower Miocene sandstones from offshore Louisiana have an average composition of quartz = 86%, feldspar = 12%, and rock fragments = 2% (Q_(86)F_(12)R2)- Feldspar and rock-fragment content increase southward as source areas shifted to include volcanic and carbonate rocks. Composition of samples from offshore Texas ranges from Q_(67)F_(24)R9 in the upper Texas coast to Q_(58)F_(24)R_(19) in the lower Texas coast. Lower Miocene sandstones from the onshore Burgos Basin, northern Mexico, have an average composition of Q54F22R23, whereas sandstones from the Veracruz Basin, southern Mexico, contain the highest proportion of rock fragments, Q33F12R55. Main diagenetic events in quartz-rich lower Miocene sandstones in Louisiana were mechanical compaction and precipitation of quartz cement. Compactional porosity loss increased to the south with increasing rock-fragment content. Calcite is the most abundant cement in the south and is strongly related to reservoir quality loss.At moderate burial depths, the best reservoir quality occurs in quartz-rich sandstones in Louisiana and decreases with increasing lithic content in Texas and Mexico. Porosity is higher in Louisiana and upper Texas than in lower Texas and Mexico at all depths and temperatures, but at depths >5 km and temperatures >175 °C, porosity differences are lessened. The lower Miocene sandstone trend in the western Gulf of Mexico from Louisiana to Mexico is an example of the importance of variation in detrital mineralogy as a control on diagenesis and reservoir quality.