Abstract

The Upper Permian (Guadalupian) Grayburg Formation at Maljamar field (Northwest Shelf, Permian Basin) consists of cyclically interbedded shallow-marine carbonate facies and restricted-marine to nonmarine sandstone facies. Carbonate facies were deposited during relative sea-level rises, whereas sandstone facies prograded across the Grayburg platform during and after relative sea-level falls. Multiple relative sea-level fluctuations resulted in deposition of 1-12 m thick shallowing-upward cycles. These cycles typically consist of basal, subtidal carbonate facies that are overlain by progressively shallower-water carbonate and siliciclastic deposits. Longer-term accommodation trends are recorded within the Grayburg Formation as four èhigh-frequency sequencesí (HFSs). Each HFS is defined from stacking patterns of lithofacies within individual meter-scale cycles. The four HFSs are arranged into a retrogradational, then aggradational to slightly progradational stacking pattern, which reflects an even longer-term accommodation trend that is recorded by the entire Grayburg Formation (i.e., the èGrayburg composite sequenceí).

A subaerial exposure surface commonly separates carbonate strata from overlying sandstone beds within cycles from updip parts of the study area. In contrast, contacts typically are gradational between carbonate and sandstone facies in downdip cycles. These relationships indicate that only updip parts of the Grayburg platform were subaerially exposed during lowstands, while downdip parts often remained submerged. During most cycle-scale lowstands, relative sea level generally fell to a position near the antecedent ramp crest that was created during the previous meter-scale cycle. At the HFS-scale, this resulted in deposition of thick, apparently ènon-cyclical,í grainstone facies near the southern limits of Maljamar field.

Nonmarine to shallow subtidal sandstone facies are the most important Grayburg reservoirs at Maljamar field. Nonmarine sandstone facies were deposited across the inner Grayburg platform during high-frequency lowstands and effectively decreased the space that was available for carbonate sedimentation during ensuing relative sea-level rises. Subtidal carbonate facies within individual Grayburg cycles most likely would have been significantly thicker and overall stratal architecture would have been different if sandstone facies were not deposited on the Northwest Shelf.

Amalgamation of sandstone- beds also makes it difficult to recognize meter-scale cycles in updip parts of the Grayburg platform. As a result, updip sandstone-dominated cycles are apparently thicker than average near sequence boundaries. These cycle-thickness trends are opposite to those expected for inner-platform stratigraphies that are composed entirely of carbonate facies. For èpureí carbonate successions, individual cycles tend to become thinner during the regressive phases of long-term accommodation trends. For the Grayburg platform, however, thick, amalgamated, nonmarine sandstone deposits are the most regressive strata (deposited when the inner platform was subaerially exposed for extended periods of time and high-frequency relative sea-level rises were unable to flood the inner platform). Amalgamated nonmarine sandstone beds create apparently thick cycles that might be erroneously interpreted as reflecting an increase in long-term accommodation, when in actuality, they record very low-accommodation conditions across the inner platform.