Integrated Reservoir Characterization of Cambay Shale for Shale Gas Resource Estimation in Ankleswar Area, Cambay Basin, Gujarat, India.

Abstract

Unconventional gas shales are described as organic-rich, fine grained reservoirs and are typically dominated by clays. The shale gas reservoirs have received great attention in the past decade, because of their large reserves as well as recent technical advances in developing these resources. Accordingly, there are increasing demands to understand the petrophysical geochemical, and mechanical properties of these gas shale rocks. The evaluation of hydrocarbon generation potential and maturity is primarily determined through geochemical analysis of source rock and its oil/gas potential. The present study carried out to evaluate the potential and maturity of the Cambay shale of Ankleswar area in south Cambay Basin in India. The study area lies in south of the northern bank of the Narmada river and northeast of the Ankleswar oil field within the Narmada tectonic block. The area of study covers 160 sq.km in which Cambay Shale formation varies in thickness from 25 m in east to 700 m in west. The top depth of the Cambay shale varies from 1100 m to 2200 m in the study area. The main objective of the research work is to understand and establish the present status of Cambay Shale formation (CSF) in Ankleswar area of Cambay basin, India for its gas potential and producing capabilities. To achieve the objective of the study detailed analysis was carried out with all the available data sets i.e. well logs, seismic data, core & drill cuttings and geochemical data of five vertical wells namely Well # 1,2,3,4,5, which were drilled through Cambay shale section in this study area. The petrophysical, geochemical and geomechanical study of a shale gas reservoir is useful in identifying the mature intervals which can be perforated and fractured effectively. Whole core and drill-cutting samples were collected from different depths and a detail analysis was carried out based on well logs, burial history, seismic, geochemical, mineralogical and optical data. Various methods i.e. rock-eval pyrolysis, optical methods, well log data for ∆ log R techniques and 1D modelling (burial history study by Petromod software) were used for analysis and characterization. The rock-eval data of core sample shows the maximum total organic carbon (TOC) of 5.14 wt % while the ∆ log R techniques show max TOC of 4.85 wt %. The log derived VRo is 0.86 % at 1935 m where as the VRo from core sample is 0.84 % at 1941 m which indicate the maturity of CSF in the study area lies within the oil window. The 1D modeling results show VRo value 1.20 % at 2150 m MD indicating wet gas maturity level in deeper part (> 2150 m) of the Well#1. The Cross plots between the TOC vs S2/S3, T max vs HI and VRo vs LOM shows the CSF in the study area falls in the mature zone from oil to wet gas window. The oxygen index (OI) vs. hydrogen index (HI) is shows Type-III kerogen which is more favorable for gas generation. Modeling study indicates presence of highly matured organic rich layers within the CSF of the study area. Integrated results of various geochemical analyses indicate that the maturity of the Cambay shale in the study area falls in oil generation window in shallow depths whereas maturity of the deeper depths ranges from wet to dry gas generation window Semi quantitative bulk-rock X-ray diffraction (XRD) analyses were carried out on drill cutting samples from wells. The major mineralogy of the study shale is quartz, kaolinite, mixed clay, pyrite with minor calcite and halite. Brittleness Index (BI) was estimated by using the mineralogy data following the equation of Jarvie (2007). Results of Fourier transform infrared (FTIR) spectroscopy and petrographic observation also confirm the presence of the same minerals in CSF. Geomechanical properties i.e Young’s modulus, Poisson's ratio, brittleness of Cambay shale was derived from sonic logs and validated with the predicted brittleness value from mineralogy data. Modeling of the brittleness index (BI) of the total study area was performed in Petrel software using P- impedance seismic volume as a secondary input. In the Cambay shale zones static Young’s modulus (YME) varies between 1.4 – 7.54 GPa whereas the Poisson’s ratio (PR) ranges 0.10-0.43. The semi quantitative mineral determination method was used and it was carried out based on intensity of the major XRD peaks preset in the sample and the present work was identified the major mineralogy present in the sample. Based on the mineralogy calculated Brittleness Index (BI). It was concluded that Brittleness Index (BI) varies from 0.44 to 0.75 which indicates high brittle rocks. The well log analysis was carried out from Geolog-6 software and the output results of porosity, water saturation, TOC, VRo, brittleness index were transformed in to seismic volume by using the VSP check-shot data in Petrel 2014 software. Established the relation between seismic P-impedance with log derived result values and established relation equations for distributing in total study area. All the model output results were validated with 1D modeling by using PetroMod-2013.2 software and core data. Integrated all the individual models and filtered minimum low- best high cases with desirable reservoir parameters established from producing shales of USA. Sweet spot areas and volumes were identified on the basis of the cut-off values of various parameters: As per integrated modeling, the total volume calculated on low-best-high cases of Cambay shale rock, which satisfies the desirable parameters in terms of shale gas exploration and exploitation within the study area in Cambay basin, India. Based on the present study, a shale gas well location was proposed for shale gas exploration and exploitation within the study area