--> Carbonate Cementation in Clastic Reservoirs: Browse Basin, Australia

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Carbonate Cementation in Clastic Reservoirs: Browse Basin, Australia

Abstract

In sandstones, carbonate cementation can be one of the key controls on reservoir quality. An example is the Plover Formation, Browse Basin, Australia. In order to predict reservoir quality distribution related to carbonate cementation away from wellbores, a workflow has been developed and utilized for the Browse Basin that potentially can be applied to other clastic reservoirs. This workflow includes the following steps: (1) Determine the types of carbonate cements, including mineralogy, origin, and timing based on petrographic observations and geochemical analyses; (2) Determine the distribution of carbonate cements by linking their occurrence to facies and structural elements; (3) Apply outcrop analogues to constrain the geometry of carbonate cemented geobodies. Results indicate that calcite cements have significantly impacted reservoir quality in the Plover Formation. Two types of calcite cements were observed: early and late calcite cements. Early calcite cement pre-dates significant physical compaction, filling primary and secondary moldic pores as well as early fractures. The late calcite cement is post-compaction, occluding primary pores and late fractures. The amount of carbonate cements ranges from 16% to 42% for early calcite, 1% to 20% for late calcite. Both calcite cements mostly occur in sandstone facies that contain carbonate grains. Facies without carbonate grains typically are devoid of carbonate cements. For early calcite cement, the δ18O and δ13C isotopic values range from −5.3 to −9.7‰ VPDB, and +0.4 to −7.1‰ VPDB, respectively. For late calcite cement, the δ18O and δ13C isotopic values range from −9.5 to −13.0‰ VPDB, and +1.2 to −2.7‰ VPDB, respectively. Fluid inclusion homogenization temperatures indicate low precipitation T for early calcite (<50°C) and high T for late calcite (68°C to 132°C). Tm-ice measurements indicate freshwater for early calcite (0.0°C), and brackish to seawater for late calcite (-1.0°C to −1.9°C). Integrated study of petrography, stable isotopes, and fluid inclusions indicate that early calcite was precipitated from meteoric waters, whereas late calcite cement was precipitated from high-T burial-modified seawater. Both calcite cements are patchy and concentrated in intervals with variable thickness cutting across stratigraphy based on core and petrographic observations. Outcrop analogues suggest carbonate cements commonly occur as irregularly distributed concretions, in the scale of cm to hundreds of meters.