| Popis: |
Summary Onshore Jurassic Cotton Valley sandstones are gas and oil reservoirs in the northern Gulf of Mexico basin. Reservoir quality varies significantly across this region, increasing from west to east. Petrographic analysis was conducted to determine the influence of detrital mineral composition, texture, diagenesis, and thermal history on reservoir quality. Chlorite and illite clay flakes oriented parallel to detrital grains form partial to complete clay rims in most Cotton Valley sandstones. In east Texas and Louisiana, clay rims are discontinuous and cover only a small percentage of the grains. Abundant quartz cement precipitated where clay rims were absent. Cotton Valley sandstones in Mississippi contain more continuous clay rims that inhibited quartz cementation. As a result, porosity is higher in Cotton Valley sandstones from Mississippi (average 15.7%) than it is in sandstones from east Texas (5.7%) or Louisiana (4.4%). Geometric mean permeability is also higher in Cotton Valley sandstones from Mississippi (7.3 md) than in sandstones from east Texas (0.02 md) or Louisiana (0.06 md). Better reservoir quality of onshore Cotton Valley sandstones in Mississippi than in east Texas and Louisiana resulted from a combination of factors: coarser grain size, more continuous clay rims on detrital grains, and lower thermal maturity. |
