ABSTRACT: Suspension- and Current-Deposit Reservoirs in the Delaware Basin: Trends and Cycles in Siltstones of the Permian Bone Spring Limestone

John C. Lorenz, Larry L. Brooks

Cores show that siltstone to very fine-grained sandstone oil reservoirs within the Permian (Leonardian) Bone Spring Limestone are composed of submillimeter- to centimeter-thick sedimentation laminae. Blanketing of small-scale topography suggests that the laminae were deposited over wide areas as sediment settled vertical to the sea floor. Superimposed on the event laminae were (1) background sedimentation of dolomitic, organic-rich, mudstone, (2) invertebrate burrowing, and (3) reworking by gentle bottom currents. Currents produced wispy ripply bedding and starved ripple forms that were draped by later deposits. Paleoflow was subparallel to the basin margin. Several sedimentation patterns occur within the formation. Three clastic intervals 25-50-m thick are interbedded wi h dolomitic mudstones of similar thickness. The clastic intervals are composed of three to six siltstone beds, each up to 25-m thick. Upsection within the beds increases in event-laminae thickness, bioturbation, and current reworking. Other reports have suggested that these are turbidite-fan deposits, but locally, paleocurrent orientations and the lack of diagnostic assemblages and sequences of sedimentary structures argue against this interpretation. Rather, some of these deposits compare favorably with the few existing sedimentologic descriptions of Quaternary dust storm deposition in marine basins. The siltstone beds may also record deflation of the exposed adjacent shelf during lowered sea level.

Very fine grain size and extensive carbonate cementation produce 5-10 µd permeability and 4-15% porosity. Sedimentary laminations cause significantly reduced vertical permeability where they are not disrupted by bioturbation, but permeability may be enhanced by natural fractures. Some reservoir thickenings are attributed to the formation of giant ripples by bottom currents, dictating a different exploration rationale than the turbidite-fan model.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990