An Integrated Geological Model for the Greater Cheleken Area Central Caspian Basin, Turkmenistan; Complex Synsedimentary Transcurrent Faulting and Compartmentalization in Plio-Pleistocene Clastic Reservoirs

Abstract

Abstract The Greater Cheleken Area (GCA) reservoirs, offshore developed by Dragon Oil, are situated on a complex NW-SE dextral transcurrent zone which separates the Northern from the Southern Caspian Basin (SCB), hosting numerous hydrocarbon fields in thick, highly faulted Pliocene-Pleistocene clastic successions of the paleo-Amu-Darya Delta System originating in the N-Balkan Fold belt. We present a new model for the geological evolution of the area, based on elaboration and interpretation of 3d seismic data (643 km2), well logs (ca. 300 wells), and a detailed remote sensing study of the Cheleken peninsula, integrated with available published data. The Apsheron-PreBalkan Fault Zone (APBFZ) separates the Southern from the Northern Caspian Basin, and connects to the SE with the Ashgabat ("Main Kopet-Dagh") Fault Zone (AFZ) which separates the Kopet-Dagh fold belt (SE) from the Balkan fold belt and Turan Block (N). These dextral shear zones show activity from at least the Miocene onwards, evidenced by a minimum of several km's post-Mid Pliocene lateral offset. In our new model a regional dextral left stepping transpressional/reversal bend between APBFZ and AFZ shows internally alternating arrays of interconnected transpressive/transtensional (R; E-W) and transpressional (P; ENE-WSW) positive hydrocarbon bearing flower structures of different types (Lam, Zdhanov-Jygalybek-Yangummez, Cheleken-Aligud-Daghazil, Goturdepe, Barsagalmez, Burun, Nebit-Dag, Urundzhuk, Monzukhly, Karatepe, Kyzylkum, Gumdag, etc.). The GCA shows 3 typical structures: Cheleken-Daghazil (positive Palm-tree flower shape, sigmoidal map shape), Zhdanov (asymmetrical top graben, deeper thrust-block, negative Palm-tree shape), and Lam (3 segments, semi-circular-elliptical-rhomboidal map shapes, positive Tulip-shaped push-ups). Hydrocarbons migrated upwards from the M. Miocene Maikop Fm source rock. Mud-volcanoes dissect the structures and show activity up to present with expulsion of hydrocarbons mixed with deeper Cretaceous and Paleo-Neogene sediments. The major fault structures reach the sea floor/land-surface, and erosion levels reach the mid Pliocene CH2 unconformity; the latest inversion along the structure has taken place during and after Mid Pleistocene times. Unconformity-bound sequences of alternating sandstones / shales with limited limestones, conglomerates, and evaporates (deposited in shallow marine, delta front, delta plain, alluvial, and lacustrine sedimentary environments): Uppermost Miocene - Lower Pliocene Red Series (Lower, Middle, Upper Red Series), Upper Pliocene Akchagyl Fm, Lower Pleistocene Apsheron Fm, and Upper Pleistocene – Holocene, comprise twelve productive levels (ca.1000 – 4000 mssl in the GCA). Hundreds (in subsurface) to thousands (in outcrop) of mappable faults (transpressive / transtensive; m. – 100s m. scale) are connected to deep-seated shear zones, and show fault-sealing (highly compartmentalized reservoirs with vertical and horizontal separations, varying in shape and size both in vertical and horizontal sense), continuous Pliocene-Pleistocene syn-sedimentary activity (sediment thickness heterogeneities, pinch-outs, lateral facies changes, channel structures), and inversion structures. The multi-scaled tectonic and sedimentary complexity described for the GCA provides fundamental insights in what constitutes a major challenge on the different strategies in the Production of the Reservoirs, and opens a gateway to unlock important new Exploration and Appraisal potential.