Reservoir Quality of Lower Miocene Sandstones in Lowstand Prograding Wedge Successions, Vermilion Block 31, Offshore Louisiana

ABSTRACT Sandstones of the upper lower Miocene Robulus L zones are currently major deep exploration targets in the mature shelf area in the Vermilion and South Marsh Island Areas, offshore Louisiana. We examined the petrography of the Robulus L 2 and 5 sandstones from two wells in Starfak field, Vermilion Area, to determine controls on reservoir quality. Whole cores from depths of 14,290 to 14,912 ft sampled lowstand prograding wedge deposits in the Robulus L 2 and 5 zones. Bioturbated very fine grained to fine-grained sandstones and interbedded mudstones coarsen upward into clean, fine- to medium-grained, crossbedded sandstones. The sandstones are interpreted as having been deposited in prodelta to delta-front depositional environments in fourth-order prograding wedges. The delta-front sandstones are overlain by potentially sealing transgressive mudstones. Sandstones are subarkoses having average composition of Q82F12R6. Good reservoir quality has been preserved in most clean proximal-delta-front sandstones. Core-analysis porosity ranges from 12 to 27% (average 21%) and permeability from 0.3 to 766 md (geometric mean = 70 md). Average volume of primary porosity estimated from thin sections is 17%, and secondary porosity is 4%. Calcite (average 5%), quartz (4%), and chlorite (2%) are the most abundant authigenic minerals. A few fossiliferous zones in the Robulus L 2 sandstone are tightly cemented by as much as 33% calcite filling primary pores and replacing grains. Secondary pores result mainly from dissolution of feldspars; calcite-cement dissolution was minor. Depositional environment, not diagenesis, is the main control on reservoir quality in these deep, potential reservoir targets. Testing of lower Miocene sandstones has been limited, but Starfak well Texaco No. 6-31 produced 1.5 MMbbl of condensate and 24 MMcf of gas from the Robulus L 2 sandstone between 1983 and 1992.