Integrated Subsurface Analysis for Conceptual Geological Model Development of Basement, Ingoli Field, Cambay Basin, India

Abstract

As the exploration for hydrocarbon resources continues to move into nontraditional areas, geologists are targeting reservoir rocks not usually associated with typical sedimentary-basin settings. One such group of rocks are volcanic rocks. In drilling 10 wells in the Deccan volcanic province in the Ingoli field in the Cambay basin, all wells encountered thick sections of Deccan basalt with intermittent fine-grained intratrappeans. However, only three of the wells produced hydrocarbon. In this formation, conventional resistivity was unable to distinguish fluid saturations, so defining the hydrocarbon-bearing zones was not possible. A nonconventional and integrated approach successfully characterized the formation and the reservoir. Using image logs, together with mud logs, we defined the facies that could be productive in this basalt formation. All the geological information was used in material balance analysis to estimate possible ranges of original oil in place (OOIP). Well data, particularly borehole images, seismic attribute interpretation, geomechanical analysis, and reservoir and production data were used to understand the reservoir system, characterize the facies, and provide guidelines to delineate the reservoir. Significant uncertainties had to be addressed to ensure successful exploitation of significant remaining oil in place. A number of multibean productivity tests and pressure buildup tests were acquired. Pressure transient analysis of these well test data, when incorporated with borehole image interpretation, provided important insights in understanding the productive reservoir facies. A conceptual geological model for the Ingoli basement reservoir is developed by integrating all the boreholederived geological information with seismic and reservoir analysis. The conceptual geological model forms a basis to consider the next steps of effectively exploiting the Ingoli field. Introduction Exploration and exploitation of hydrocarbon from unconventional reservoirs pose new challenges to subsurface geologists where conventional technology and proceses may not all be applicable. Worldwide basement rocks have indicated significant oil resources, but only a small percentage (Byeonggoo Choi, et. al., 2008) have been exploited. The Deccan basement of the Ingoli field of the Cambay basin has been identified as hydrocarbon bearing volcanic reservoir. Gujarat State Petroleum Corporation (GSPC) drilled ten wells through the Deccan volcanic in the period 2003-2004 in the north-south trending ridge, called the PK high (See details in Geophysics Section), in the Cambay basin (Fig. 1). All the wells encountered thick siliciclastic sequence underlying Deccan basalt with intermittent fine-grained intratrappeans. The prime target here is fractured and weathered basaltic rock. The general stratigraphy (Fig. 2) outlines the source, reservoir, and caprock present in this part of the basin. The uplifted older basement block proved to be a potential reservoir against younger Cambay shale, which is a major source rock. The structural framework in this region is characterized by three fault trends, namely, Dharwarian, the oldest, trending north-northwest to south-southeast; Aravalli, trending northeast to southwest; and Satpura, the youngest, trending east-northeast to west-southwest. These trends define the fault system of the study area. The Dharwarian fault system is believed to be the conduit for hydrocarbon migration from source to the reservoir.