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AAPG Bulletin, V.
A megascale view of reservoir quality in producing sandstones from the offshore Gulf of Mexico
A publicly available data set has been examined for relationships between average values of porosity, permeability, depth, temperature, pressure, thickness, age, and play type for 11,833 sandstone reservoirs, mostly of Miocene age and younger, from the United States offshore Gulf of Mexico (GOM). Porosity shows wide scatter as a function of burial depth, but the median (P50) porosity trend decreases smoothly with depth. The GOM trend has much higher porosity for the given depth than the P50 trend of sandstone reservoirs worldwide, reflecting rapid sedimentation rates and young ages of GOM reservoirs, most of which have spent relatively little time at temperatures more than 80C, where quartz cementation becomes active. Multivariate regression analysis shows that porosity is best predicted by temperature (r2 = 0.40), with the fit improved slightly by adding age and then depth (r2 = 0.44). Arithmetic average permeability (represented by its logarithm) shows a correlation of maximum and P50 trends with porosity. GOM P50 permeability lies 0.2–0.4 log units below the P50 trend for sandstone reservoirs worldwide, probably reflecting very fine grain size of most GOM sands. Water saturation can be used to calculate the effective (petroleum-filled) porosity of each reservoir, which shows strong correlation with permeability. Grouping the reservoirs by chronozone reveals regular trends of decreasing average porosity and permeability with increasing age, reflecting increasing average depth and temperature with age. Porosity and permeability functions representing depositional sand quality show only subtle differences between different age groupings and play types. The results presented here can be useful for specifying realistic distributions of parameters for both exploration risk evaluation and reservoir modeling.
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