Geological Reservoir Modelling in an Unconventional Play Setting: Appraisal of the JinQiu Block, Sichuan Basin, China

Abstract Fluvial-lacustrine deposits of the Xujiahe Formation are the primary reservoir target for an ongoing appraisal project in the northwest Sichuan basin. The late Triassic clastics are comprised of six members T3×1 to T3×6. The amalgamated channel sandstone sections of the T3×2 and T3×4 members may form an "inefficient" Basin-Centered gas play charged from downdip source rock while the calcarenites, coaly shale, and calcareous siltstones of the T3×3 and T3×5/6 are possibly charged by local source rock. Static modelling helped to improve understanding of production history for un-fracced vintage wells and early production behavior of newly drilled appraisal wells. Well sector models (scale of ~1×1 km) were created for this purpose. The ability to predict areas with low water production is crucial for the economic feasibility of Xujiahe development. Water saturation versus porosity cross plots provided key input to saturation models in static models. The cross-plot method tentatively helps predict well productivity but further calibration is necessary. Introduction The Xujiahe formation consists of sequences of coaly calcareous siltstones and shales alternating with high Net/Gross (N/G) quartzo-feldspathic arenites. In the study area, the Xujiahe Formation ranges from 900–1300m in thickness, deepening and decreasing in N/G towards the NW. These changes reflect a facies change from stable platform in the SE Sichuan basin to a foredeep to the NW. The Xujaihe formation is stratigraphically subdivided into six members T3×1 to T3×6 across the Sichuan basin. In the study area the uppermost T3×5 and T3×6 members consist of interbedded shales, thin coals, calcareous silstones and calcarenite sandstone. Although separated elsewhere in the basin, in the study area the T3×5/6 are combined because no clear boundary is recognizable. Xujiahe members T3×3, T3×2Lower and T3x1–3 are similar in lithology to the T3×5/6. The depositional environment of these coaly intervals is interpreted as shallow lacustrine/marsh environment with ephemeral meandering rivers. Calcarenite sandstones occur as isolated 10–50m thick intervals and are difficult to correlate. Calcarenites are interpreted to erode underlying shales, siltstones and thin coals, an assertion supported by limited 3D seismic. Calcarenite log porosity ranges from <3–5%. Calcarenite sandstone intervals do not contain observable wet zones.