Catherine L. Price¹, James M. Mazzullo¹
(1) Texas A&M University, College Station, TX

Abstract: Depositional lithofacies, cycle stacking patterns, and reservoir heterogeneity of Permian (Guadalupian) Grayburg Reservoirs, Means Field, Andrews County, Texas

The Grayburg Formation of Means field, Andrews County, Texas, consists of mixed siliciclastic and carbonate sediments deposited in three general platform environments: subtidal open shelf; subtidal and intertidal shallow shelf; and supratidal flats and sabkhas.

The subtidal open shelf lithofacies consists of peloid-fusulinid dolowackestone/dolopackstone which accumulated along the outer portion of the platform, generally below fairweather wave base. The distal shallow shelf, shoal, restricted shallow shelf, and shoreface lithofacies were deposited in subtidal and intertidal shallow shelf environments. The distal shallow shelf lithofacies consists of skeletal, peloidal dolowackestone/dolopackstone; shoal lithofacies include ooid dolopackstone/dolograinstone and oolitic, very fine-grained sandstone. The restricted shallow shelf lithofacies is composed of peloidal dolowackestone/dolopackstone/dolograinstone. Shoreface lithofacies include laminated quartzose dolostone and dolomitic coarse-grained siltstone/very fine-grained sandstone. The supratidal environment includes carbonate tidal flat and coastal sabkha lithofacies. The carbonate tidal flat lithofacies contains the coated grain subfacies and the dolomudstone subfacies. The coastal sabkha lithofacies consists of reddish-brown argillaceous coarse-grained siltstone/very fine-grained sandstone and nodular/bedded anhydrite.

Vertical repetition of lithofacies was used to identify small-scale shallowing-upward cycles. These shallowing-upward cycles are related to high-frequency low-amplitude sea-level fluctuations that affected platform sedimentation. Carbonate sedimentation dominated the platform during sea-level highstands, whereas prevailing tradewinds transported siliciclastic sediments onto the platform during sea-level lowstands.

Original depositional successions were modified by diagenesis to create highly layered, heterogeneous reservoirs with low recovery efficiencies. However, an understanding of the processes controlling the distribution of remaining mobile oil may significantly improve recoverability in these reservoirs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana