Carbonate deposition and hydrocarbon reservoir development at the Precambrian–Cambrian boundary: The Ara Group in South Oman

Abstract

The Ediacaran–Early Cambrian Ara Group in the South Oman Salt Basin consists of six evaporite–carbonate cycles (A0/A1 to A6) that record tectono-eustatic sea-level changes. The A4 cycle developed at the Precambrian–Cambrian boundary (∼ 542 Ma). It forms a single depositional sequence with evaporites deposited in a lowstand systems tract (LST). Overlying carbonates represent an open-marine shallowing-upward ramp succession that developed in transgressive (TST) and highstand systems tracts (HST). The low-energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high. The TST facies include sulfates, evaporite–carbonate laminites, and organic-rich carbonate laminites that record initial flooding, deepening of the basin, and establishment of an outer ramp depositional environment. Carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity. Subsequent HST facies comprise mostly fine-grained clastic carbonates and stromatolites that formed in middle and inner ramp settings. These facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp. Sediment was redistributed in suspension, by muddy turbidity currents, muddy debris flows, storm and shallow-water currents. During the late HST, carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying LST evaporite unit. A combination of strong tectonic subsidence and transient flooding caused significant shallow-water evaporite deposition to occur not only down dip, but also on top of the former carbonate platform, where several hundreds of meters of evaporites accumulated. The transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit. The presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties. Distribution of organic material and carbonate mud can be linked to specific environmental conditions (low physical and biogenic disturbance of sediment and stratified water mass) and the sequence stratigraphic position (low flux of fine-grained carbonate during TST). In contrast, diagenetic evaporite formation has largely degraded reservoir quality in porous shallow-water facies near the top of the A4C.