It is commonly accepted that deep marine reservoirs are dominated by submarine channels and channelized lobes. Submarine channels can be broadly classified as gravity-flow channels and leveed channel complexes. While gravity-flow channels are characterized by strong textural heterogeneity, thin-bedded channels and associated levees show rapid lithological changes at cm scale, well beyond the vertical resolution of standard logging techniques. This paper highlights a technique for enhanced lithofacies prediction in a deep-water clastic turbidite reservoir, offshore India, principally using resistivity images and spectroscopy logs. We also show how an integration of the micro-resistivity images together with other well log data enabled a more accurate estimation of gas in place compared with using only conventional logs in this ?low resistivity pay?, thin-bedded reservoir. The micro-resistivity images, together with the spectroscopy log derived dry weight lithologies were used to generate an enhanced lithofacies using a novel lithofacies processing and classification scheme. The system uses the dry weight mineralogical output from the spectroscopy data, and with a corresponding set of rules to create dry-weight mineralogy-based lithofacies. This output is further sub-divided to a finer scale using calibrated micro-resistivity image data. Advanced image processing and interpretation enabled an estimation of reservoir heterogeneity. A heterogeneity index is calculated from the percentile resistivity distribution of the image spectrum. Since the heterogeneity index is related to a distribution of resistivities around the borehole, it is independent of the absolute resistivity values. The integration of these results also reinforce the fact that lithology and texture are equally important as the petrophysical characteristics of the reservoir units and provide important input for lithological and textural heterogeneity during advanced formation evaluation. As continuous coring is not always feasible due to its innumerous cost, borehole image logs and spectrometry data, when calibrated with core, provide sufficient resolution and quantification for a continuous and detailed facies description.
Deepwater plays increasingly are the focus of hydrocarbon exploration and development in recent years, in India. The principal exploration targets in such a geological setting are submarine channel systems and channelized lobes (Fig.1). The Krishna-Godavari (KG) basin is located on the east coast of India in the State of Andhra Pradesh, and covers an approximate area of 100,000 sq. km. The stretch of sedimentary tract comprises a range of depositional environments comprising coastal plain, delta, shelf-slope apron, deep-sea channel and deep marine channel and fan complex (Bastia et al., 2006). The depositional systems resulting from the interplay of dynamics of Krishna and Godavari delta progradation, gravity-induced processes and relative sea level changes gave rise to mixed deepwater sand-mud system. These Tertiary deepwater clastic sediments are characterized by a wide variety of mass flow processes, resulting in complex, irregular and often disorganized lithofacies distribution. While gravity-flow channels in this area are characterized by strong textural heterogeneity, the thin-bedded leveed channels and associated levees show rapid lithological changes at cm scale, well beyond the vertical resolution of standard logging techniques, and reservoirs often remain underestimated.
