Facies Architecture, Channel System and Paleoflow Pattern of a Young Sandstone Reservoir using Borehole Resistivity Images

Abstract

Abstract The Miocene age shallow unconsolidated sandstone formation in Kuwait contains viscous oil whose distribution is primarily controlled by depositional facies and their diagenetic modifications. This study integrates core-calibrated resistivity images with openhole logs to obtain high-resolution facies logs with continuous array of oriented depositional structures, facies types, and environment-related facies associations with paleocurrents measurements. These are combined in defining reservoir zonation, geometry, spatial distribution and paleo-geographic evolution during its accretion across the field. The formation is characterized by variable grain sizes, sedimentary structures and diagenetic cementations. The sandstone succession is punctuated by scouring surfaces and separated by discrete mudstone intervals mixed with scattered bioclasts. The upper section of the formation encompasses two main oil-producing sandstone intervals (S1 & S2 reservoirs), separated by a mudstone unit (Mid Shale) and sealed on top by marine Cap Shale unit. Each sandstone interval consists of lower sheet-forming stacked horizontal-and cross-stratified sandstone units (S1B & S2B) and upper lenticular sand-prone bodies (S1A & S2A) that laterally truncate or intermingle with units of sandy mudstone and argillaceous sandstone (S1 Shale & S2 Shale). The observed physical and biological features, paleocurrents data and vertical hierarchy of recognized sandstone and mudstone facies suggest the oil-bearing sequence formed as fluvial to upper delta-plain deposits that were accreted by repeated lateral shifting, and an overlapping of fluvial channels, distributary/abandoned channels, crevasse splays and inter-distributaries lakes or bays of estuarine origin. The active fluvial-and distributary channels were generally flowing from SW to NE, and locally due E and SE, and largely discharged from S. The morphological parameters reveal that the fluvial channels migrated laterally and bifurcated downstream, and fluctuated across a low sinuous channel belt. With continuous base level rise, the area was drained entirely by transgressive brackish-water lacustrine or bay events depositing the widely distributed Mid-Shale and Cap-Shale units.