Well log evaluation of the gas-bearing reservoirs in the Bombay offshore basin, Gulf of Cambay, western coast of India

Abstract

Present study aims to locate potential reservoir zones in the Paleocene, Eocene and Oligocene sequences by analysing well log data of nine vertical wells in the exploration block MB-OSN-2004/1 in the Bombay offshore basin, Gulf of Cambay, India. The geophysical logs such as gamma-ray (GR), caliper (Cali), resistivity (LLD, LLS, and MSFL), photoelectric effect (PEE), neutron porosity (NPHI), and density (RHOB) are used to study the petrophysical characteristics of the identified reservoirs in this field. The relevant information from other sources like available report, mud log, and other open hole wireline log data, and local knowledge about the field are integrated with the petrophysical analysis. The cross-plots and mud log data show that the wells mainly consist of three types of lithology: shaly-sand, shale, and limestone, however, coal layers and pyrite are present at some places. Cross plots of thorium and potassium (from spectral gamma-ray log) show that the type of clay is mixed-layers of illite, montmorillonite and kaolinite in the Pipavav and Panna formations. Three of the nine drilled wells C-47-1, MTC-1, and MB-3-1 are identified by the petrophysical investigation as gas-bearing, principally in the Pipavav and Panna formations, with saturation ranging from 55% to 60% and porosity varying from 10% to 40%. Gas-bearing layers’ thickness varies from 5 to 40 m and are primarily composed of clastic sediment with mixed carbonates in some locations. The remaining wells are water-bearing because the wells penetrating the silt formations are abundant in clay minerals, which keep the reservoir from releasing fluid and decrease its permeability. In addition, gas cannot rise because impervious shale layers are present in the columns of sand and calcareous sand.