Distinguishing between eogenetic, unconformity-related and mesogenetic dissolution: a case study from the Panna and Mukta fields, offshore Mumbai, India A.J. BARNETT 1 *, V.P. WRIGHT 2 , V.S. CHANDRA 3 & V. JAIN 4 1 BG Group, 100 Thames Valley Park, Reading, RG6 1PT, UK 2 PW Carbonate Geoscience, 18 Llandennis Avenue, Cardiff CF23 6JG, UK 3 Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, UK 4 BG India, BG House, Lake Boulevard, Hiranandani Business Park, Mumbai, Maharashtra 400076, India *Corresponding author (e-mail: andrew.barnett@bg-group.com) Abstract: The Panna–Mukta fields host hydrocarbons in the Bassein Formation Eocene– Oligocene ramp limestones. The pore system is almost wholly secondary, comprising microporo- sity, mouldic porosity, vugs, solution-enlarged stylolites and fractures. Although petrographical evidence points to dissolution after extensive late cementation, the presence of a high perme- ability layer close to a palaeokarstic surface at the Eocene– Oligocene boundary has raised the possibility that this secondary porosity could be related to subaerial exposure. However, the Panna– Mukta reservoirs show a strong correlation between secondary matrix porosity and sty- lolite density measured from cores. Stylolites only developed in ‘clean’ limestones lacking argil- laceous material, whereas more argillaceous limestones in the succession are characterized by dissolution seams and have poor reservoir quality. These cleaner limestones occur preferentially below the Eocene–Oligocene boundary, representing an upwards-shallowing sequence, whereas the argillaceous limestones occur further below the Eocene–Oligocene boundary in the lower part of the same shallowing-upwards sequence and in the transgressive limestones at the base of the Bassein A. This secondary porosity distribution suggests movement of corrosive fluids along pre-existing stylolites. Despite an apparent link between porosity distribution and an unconformity, secondary porosity development was mesogenetic and related to the distribution of facies that favoured stylolites that acted as conduits for the flow of corrosive fluids. The Bas- sein Formation reservoirs show unequivocal evidence of significant porosity development by mesogenetic dissolution but the exact process or processes by which such porosity creation occurs requires further work. Secondary porosity is of considerable importance to the prospectivity of carbonates and often determines the presence or absence of reservoir-grade porosity. Most limestones, by virtue of having been deposited in shallow water, are prone to secondary porosity development as a consequence of relative sea-level falls and the effects of acidic meteoric fluids produc- ing early (eogenetic) unconformity-related dissolu- tion (Tucker & Wright 1990; Moore 2001; Ahr 2008). However, secondary porosity formation due to dissolution at depth related to burial fluids is increasingly being invoked, and Wright & Harris (2013) have attempted to define such dissolution (also referred to as burial corrosion) as: porosity formation caused by fluids unrelated to recharge from the overlying land surface, or adjacent water bodies, but related typically to a confined aquifer, where the source of the fluid is ultimately from below the formation (hypogene). Recognizing the relative importance of eogenetic v. mesogenetic dissolution in generating the final preserved pore system is commonly difficult. The exact mechanisms by which burial dissolution pro- ceeds are often difficult to establish and remain con- troversial. This debate has been reinvigorated by some workers (e.g. Ehrenberg et al. 2012) who not only question the veracity of the commonly cited mechanisms of burial dissolution, but also suggest that many of the petrographical characteristics used to identify burial dissolution can be explained without invoking mesogenetic processes. Thus, the aim of this paper is to assess the relative roles of eogenetic v. mesogenetic processes in creating sec- ondary porosity in the Bassein Limestone of the From:Armitage, P. J., Butcher, A. R., Churchill, J. M., Csoma, A. E., Hollis, C., Lander, R. H., Omma, J. E. & Worden, R. H. (eds) Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction. Geological Society, London, Special Publications, 435, http://doi.org/10.1144/SP435.12 # 2015 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics at BG International Ltd on December 22, 2015 http://sp.lyellcollection.org/ Downloaded from