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Reservoir Facies Within a Basin Centered Gas Accumulation, Thrace Basin of N.W. Turkey


The Tertiary-aged Thrace Basin of NW Turkey has produced 900 BCF of natural gas from conventional fault-bounded structural and stratigraphic traps (Coskun,1997). Recent exploration has focused on over-pressured, off-structure, tight gas within a potential multi-TCF basin centered gas-condensate accumulation. Primary reservoirs are siliciclastic and volcaniclastic sandstones of the Kesan Formation and Teslimkoy Member deposited in submarine fan complexes laterally coalesced and vertically stacked to over 2000m of high net: gross section. The Kesan Fm. consists of porous, low matrix permeability sandstones and interbedded siltstones, shales and tuffs. Heterogeneity in lithology and mineralogy is attributed to multiple phases of tectonism and volcanism coincident with sedimentation during basin evolution. Full diameter core and outcrop from the Kesan Fm. provide excellent examples of fine-grained turbidite facies, the product of sediment gravity flow processes emanating from deltaic complexes to the south. Approximately 85% of sandstone facies in core are thin-to thick-bedded turbidites interpreted as various elements of submarine fan lobe and/or channel environments. Sedimentologic analysis of outcrop sections from the Kesan Fm. and Teslimkoy Mbr. serve as a template for reservoir architecture and facies heterogeneity of equivalent strata observed in subsurface core. Kesan Fm. outcrop exposures along the southern rim of the basin consist of relatively thick successions of massive-appearing (structureless) very fine- to -fine grained sandstone interpreted as turbidite lobe facies associations up to 15m thick, minimally 250m in breadth comprised of high net: gross (9:1) very fine- to - fine grained sandstone. In addition, recently conducted field work has confirmed the frequency and lateral continuity of turbidite lobe and channel reservoir facies and the prevalence of natural fracture networks. Abundant organic matter accumulations in the form of resedimented plant and algal detritus (ie. carbonaceous and kerogenous debris) has also been observed in mudstone facies, providing a source rock for hydrocarbon accumulation. Future pilot program drilling in the Thrace Basin centered gas accumulation will focus on determining reservoir geometry and resource density, key elements to optimize completion strategies in either vertically drilled wells targeting multiple stacked intervals, or horizontal wells within thick laterally continuous turbidite lobes.