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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.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California