460 JOUR.GEOL.SOC.INDIA, VOL.89, APRIL 2017 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.89, April 2017, pp.460-470 Impact of Bioturbation on Reservoir Quality and Production – A Review Joyjit Dey 1 and Souvik Sen 2* 1 Department of Geology & Geophysics, Indian Institute of Technology Kharagpur, West Midnapore - 721 302, India 2 Geologix Limited, Dynasty Building, Wing A, Level 4, Andheri Kurla Road, Andheri (E), Mumbai - 400 059, India *E-mail: souvikseniitb@gmail.com ABSTRACT Bioturbation is a typically small scale yet potentially significant geological process altering rock properties by reworking. For many years, bioturbation studies found application in exploration geology to estimate paleobathymetry, interpreting depositional environment and identifying key stratigraphic surfaces. These act as vital inputs to the geological models, for determination of source rock potential, reservoir quality and modeling of petroleum systems. Recently geologists extended the application of bioturbation studies to address production related challenges. Recognizing the bioturbation effects and incorporating them in reservoir simulation models can improve production predictions and enhanced oil recovery operations. This paper discusses bioturbation and its effects on reservoir quality, its performance and production. INTRODUCTION Sediments undergo several modifications to become source rocks, reservoirs and seals to form a petroleum system. Diagenesis includes the processes of compaction, cementation, dissolution and recrystallization. But before any of these occur, another process may significantly affect rock properties – bioturbation. Bioturbation is simply the disruption of sediments and soil by living organisms. It can take several forms, including displacement of soil by roots, burrows/bores and even footprints of dinosaurs (Gingras et al., 2011). Oil and gas industry is mostly interested in understanding the changes brought about by organisms, active near the sediment-water interface in shallow marine and marine settings. Such activities are generally vertically restricted to a meter but laterally these can cover an area of tens to hundreds of square kilometers. Understanding the behaviors of these organisms helps to characterize the-then depositional environment when sediments were deposited but still were soft enough to be deformed by bioturbation. The process of bioturbation, or the organism/sediment interaction, has a crucial impact on reservoir quality and its flow behavior. Thus in bioturbated reservoir facies, ichnology is integral to reservoir characterization (Ali et al., 2010), bioturbation is capable either to enhance or diminish the reservoir porosity as well as permeability. Ichnological analysis of a reservoir facies and subsequent classification into ichnofabrics or ichnofacies allows for characterization of reservoir properties. Bioturbation can redistribute grains and cause sorting or mixing, this physical modification of the primary sedimentary fabric causes changes in porosity, and permeability of reservoir facies. In highly bioturbated reservoir facies, bioturbation can be the first order control on petrophysical properties (Al-Hajeri et al., 2009). LIFE JUST UNDER THE SURFACE Organisms living near the sediment-water interface often leave evidences of their life styles. For example, surface expressions of sub-surface bioturbation can be discerned in the intertidal zone of a beach (Fig.1). Infaunal organisms living in the sediments (like crabs, shrimp, tubeworms etc.) can disrupt sediments in many ways. They may create tube like tunnels and shafts of varying inclination. These burrows may remain open for a period of time (Fig.2), collapse or be filled immediately with similar or contrasting sediments (Fig.3 and 4). While burrows made on a consolidated substrate have better potential to stay open for a longer span of time, those made on soft substrates are prone to be filled. Some infaunal activity can cause complete mixing of a volume of sediment but leave no detectable trace (Fig.5). For example, animals foraging in layered sediments may disrupt the substrate so completely that the layering can no longer be visible. Epifaunal organisms may not burrow or modify the sediments to a great degree, but they can also leave traces of their activities in form of furrows or other tracks (Fig.6). In the rock record, bioturbation manifests itself mainly as fossilized traces of animal activities. The study of these traces is called Ichnology. Ichnologists interpret these traces to indicate animal activities as escaping, dwelling, crawling, feeding, farming and grazing, among others. Traces may be variations or combinations of these (Buatois and Mangano, 2004; Hickey and Henk, 2007). Ichnologists use the evidence of these behaviors to characterize the paleoenvironment of a rock layer. A variety of species can produce similar structures if their activities are similar. Even a single species can produce different kinds of traces while performing different activities and traces may vary depending on the substrates (Gingras et al., 2009). Thus a basic way to interpret sedimentary rocks is to divide them into three main types of lithified sediments as unburrowed, burrowed and massive (Fig.7) (Gingras et al., 2009). This serves as the starting point for interpreting depositional conditions under which such sediments formed. Unburrowed Relatively undisturbed sediments, i.e. those with original layering intact and with little or no evidence of bioturbation, are usually ascribed to one or more following depositional environments:  Freshwater, with few deeply burrowing organisms (Loucks and Ruppel, 2007)  Anoxic settings (poorly oxygenated) (Taylor and Goldring, 1993)  Constantly shifting sediments on seafloor (Pemberton et al., 2008)  High sedimentation rates (Gingras et al., 2009)  Arid or frozen areas (Gingras et al., 2009) Unburrowed sandy sediments usually indicate freshwater deposition or shifting sedimentation. However many continental environments do exhibit trace fossils. Unburrowed fine grained sediments (silty or clay dominated) are typically interpreted as product of sedimentation in fresh water or anoxic conditions, although high sedimentation rates might yield similar result. Many organic rich source rocks (some are targets of tight oil and shale gas plays) are examples of fine grained sediments deposited in environments with low oxygen supply, as such environments are not appreciable to many 0016-7622/2017-89-4-460/$ 1.00 © GEOL. SOC. INDIA