Application of Borehole Images for the Evaluation of Volcanic Reservoirs – A Case Study from the Deccan Volcanics, Cambay Basin, India

Pal, Aditi¹, Nigel Machin¹, Shashank Sinha², Chandramani Shrivastva¹ (1) Schlumberger, Mumbai, India (2) GSPC, Gandhinagar, Gujarat, India

Gujarat State Petroleum Corporation (GSPC) drilled 4 wells through the Deccan Volcanics in the period 2003-2004 on a N-S trending ridge of block, in Cambay basin. Only one of the wells PK#2 tested oil from the top layer of the volcanics, producing 400 BOPD. Later in 2005 well PK#6 was drilled 600m to the SW but did not flow hydrocarbon in the same layer. A borehole image-based facies analysis study was conducted within the volcanics as earlier studies on basic formation evaluation; fracture studies and seismic data did not provide sufficient results for reservoir characterization. A facies classification was defined for volcanics on the basis of the occurrence of vesicles and the image logs were represented as facies logs comprising of vesicular basalt, non-vesicular basalt and volcaniclastic units.

The association of vesicular basalts with conductive fractures was identified as a unique lithotype providing the necessary porosity system for a reservoir. A correlation exercise was performed incorporating borehole image facies and geochemical logs. Markers were identified in PK#2 and PK#6 on the boundary of the basalt and volcaniclastic sections. The overall disposition of the markers in the area along a N-S section represented a step-like profile that was considered to be due to localized faulting and block movement. A brecciated zone, identified on the images, was also correlated to seismic scale faults on well PK#6. This suggested that the PK#2 area formed a local high and formed a reservoir, isolated from the other wells to the southern part of the ridge.