Application of
Borehole Images for the Evaluation of Volcanic Reservoirs – A Case Study from
the
Pal, Aditi1, Nigel Machin1,
Shashank Sinha2, Chandramani
Shrivastva1 (1) Schlumberger,
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