Sedimentary Cyclicity Interpretation in Triassic Carbonate Fields in Northeastern Sichuan Basin, China

Abstract

The study area is in the Northeastern Sichuan Basin. Regional geology has been well established with tremendous geological studies, which indicating the paleogeography in this area is characterized by shallow water carbonate platforms skirting with marine troughs. Detailed geological characterization at reservoir level varies from field to field however, due to data availability and level of research depth.

The carbonate gas fields included in this study locate on the Southwest margin of an elongated platform trending Northwest Southeast. The pay sections for the gas fields are dominated by dolomitized oolitic shoals. Multiple types of subsurface data including 3D seismic, well logs and cores were collected and analyzed to assist the detailed reservoir characterization. Seismic stratigraphy and attribute analysis indicate a progradational features of the shoal bodies from a regional perspective. The detailed log and core interpretation of many exploratory and development wells clearly reveals the cyclicity, with oolitic shoals prograding upward from East to West. The reservoir-equivalent formation on the opposite Northeastern margin of the carbonate platform is outcropped to the surface. They were carefully examined in a geological field trip to the multiple outcrop locations along a direction perpendicular to the paleo-shoreline, and a similar progradational pattern was observed on a constructed profile. The patterns of such kind are also observed in the offset gas fields, further validating the subsurface interpretation of the gas fields in the study area.

Reliable cyclicity interpretation of the reservoir plays a significant role in reservoir modeling and management. The identified sedimentary cycle boundaries are used as guiding layers to preserve internal reservoir heterogeneity. Facies and reservoir properties are analyzed and modeled by cycles. The solid understanding of internal heterogeneity can further support reservoir management, such as providing guidance to position the swell packers at potential permeability barriers for water control and placing future infill wells at locations with undeveloped quality reservoir.

In summary, multiple types of geological data including cores, logs, seismic, outcrops and analogs are integrated and analyzed collectively to extract the geological insights about the reservoirs. The resulted characterization of sedimentary cyclicity and reservoir internal architecture can provide great support to ensure the optimal field development planning and reservoir management.