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A megascale view of reservoir quality in producing sandstones from the offshore Gulf of Mexico

¹ StatoilHydro, N-4035 Stavanger, Norway; sne{at}statoilhydro.com
² StatoilHydro, N-4035 Stavanger, Norway; phn{at}statoilhydro.com
³ StatoilHydro, N-4035 Stavanger, Norway; oysteen{at}statoilhydro.com

Steve has a Ph.D. from the University of California at Los Angeles. He works on sandstone and carbonate reservoir studies for exploration and production projects.

First joining Statoil in 1986, Paul now serves as a specialist in global exploration working on basin evaluation and petroleum systems analysis. Originating from Maine, Paul received a B.S. degree from Boston College and a Ph.D. from Dartmouth College. He received the Schlumberger Medal from the Mineralogical Society and the Brindley Award from the Clay Minerals Society. Presently, Paul is preparing a popular petroleum geology book outlining the diagenetic controls on hydrocarbon discovery and production efficiency, focusing on the strong relationships between recoverable reserves and reservoir temperature, as well as implications for future energy resource management.

Øyvind received his M.Sc. degree in 1994 and his Ph.D. in 1997 in structural geology from the University of Oslo. He joined Statoil in 1997 and has been working as a production geologist and in exploration. His current research concerns the reconstruction of sedimentary basins and thermal-history modeling.

ABSTRACT

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 80°C, 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.