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Reservoir Wettability Alteration as a Key Enabling Factor for the Hydrocarbon Accumulations in the Deeply Buried Tight Reservoirs in Tarim Basin, China

Zhao, Mengjun; Liu, Keyu; Li, Yong; Liu, Shaobo; Fang, Shihu; Guo, Xiaowen; Zhuo, Qingong; Lu, Xuesong; Fan, Junjia

In a conventional petroleum system, petroleum migration and accumulation are thought to primarily occur along porous and permeable pathways and in traps with good reservoir petrophysical properties, as petroleum migration and accumulation in tight rock formations would require extraordinarily large driving force or a sustained huge hydrocarbon column. Apart from the pore-throat size, reservoir wettability also plays an important role in controlling the capillary force required for hydrocarbon migration and accumulation. This parameter is, however, rarely considered in the conventional petroleum system analysis. Wettability refers to the preference of a solid to be in contact with one fluid rather than another. It is an important parameter that is often manipulated by petroleum engineers to improve oil recovery. For clastic sedimentary systems, reservoirs are generally regarded as strongly water wet. Organic matter associated with fine sedimentary rocks turns to be oil wet due to its affinity to oil and may become strongly oil-wet once hydrocarbons were produced. Reservoir wettability can also be altered from water-wet to oil-wet when the rock and oil are in contact for sufficiently long period under subsurface P/T conditions.

Recent exploration in the Kuche Depression, Tarim Basin, western China has discovered some giant hydrocarbon accumulations in tight sand reservoirs with reservoir porosity of less than 5% at depth of over 6000 m. Detailed analysis using quantitative fluorescence spectroscopy, fluid inclusion petrography, Field Emission SEM, synchrotron micro-CT imaging revealed that these tight reservoirs contain organic coated reservoir grains and residual oil/bitumen that form interconnected oil-wet network. Coupled basin modelling and charge history investigation revealed that the Kuche Depression received a widespread early oil charge prior to the regional reservoir densification and a subsequent late hydrocarbon charge which resulted in these giant accumulations. It is interpreted that the early oil charge and the presence of organic matter in the reservoirs altered the reservoir wettability and enabled hydrocarbons to migrate through the extremely tight reservoir formations with little capillary resistance with hydrocarbon migration virtually via "imbibitions".


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013