Integrated geological-geophysical characterizations of deeply buried fractured-vuggy carbonate reservoirs in Ordovician strata, Tarim Basin

Abstract

The Central Uplift region of the Tarim Basin contains a series of thick Ordovician carbonates that form fractured-vuggy hydrocarbon-rich reservoirs along an unconformity. These fractured-vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale, geometry and spatial distribution. In this paper, a series of deeply buried fractured-vuggy carbonate reservoirs are characterized on micro-, macro- and mega-scales. These characterizations are based on previous studies detailing the carbonate sedimentation, diagenesis, and tectonic processes in the study area and abundant geological and geophysical data, including core samples, thin sections, and well logging and seismic datasets, which have been obtained and upscaled using the “choosing-classification-evaluation- generalizing-upscaling” (CCEGU) method. In this area, different facies can be identified at different scales: (1) Detailed core and thin section observations performed based on the Dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro-scale (0.01 m); (2) logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro-scale (0.1 m–1 m), and the presence of electro facies in individual wells can be recognized; and (3) these 9 different electro facies comprise 6 distinct seismic facies at the mega-scale (10 m), and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties. The targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near NE-trending strike-slip faults. This study combines geological and geophysical response data from fractured-vuggy carbonate reservoirs to achieve seamless connections among micro-scale, macro-scale and mega-scale observations to effectively characterize a series of strongly heterogeneous fractured-vuggy reservoirs. This method can also be applied to characterize other deeply buried paleokarst reservoirs in the Tarim Basin, as well as other strongly heterogeneous reservoirs.