Sedimentology of Early Aptian Reservoir: Dunga Field, Mangyshlak Basin, Kazakhstan

Cazier, Edward ¹; Hein, Malene ²; Pemberton, George ³
(1)Maersk Oil, Copenhagen, Denmark. (2) Maersk Oil Kazahkstan, Almaty, Kazakhstan. (3) University of Alberta, Edmonton, AB, Canada.

Maersk Oil, and partners Oman Oil and Partex, are actively developing the Dunga field, located 50 km north of the city of Aktau, on the Kazakhstan coast of the Caspian Sea. The field is unique, in that it is a Cretaceous (Early Aptian) oil reservoir in a province of mainly Jurassic oil and gas reservoirs.

Sedimentary characteristics of the Early Aptian reservoir observed in core include: mud, silt and very fine-grained sand; organization of sand deposits into centimetre- to decimetre-scale beds, with muddy and silty interbeds, which occasionally amalgamate into meter-scale sand intervals where the best reservoir is located; pervasive soft-sediment deformation; common hummocky cross-stratification; and frequent syneresis cracks.

The trace fossil assemblage is variable in diversity, size and abundance. Cores immediately below and above the reservoir display intense and uniform bioturbation, normal trace sizes, and presence of filter-feeding burrows. In contrast, the intervening reservoir section shows a reduced trace fossil density and sporadic bioturbation, and fewer filter-feeding burrows. Furthermore, traces in the reservoir section are stunted, and reflect a stressed environment with large salinity variations and high sediment input.

Paleogeographic interpretations during the Aptian place the Dunga area within a broad embayed shelf, distant from highland areas. Analysis of the core data, and integration with log and 3D seismic data, suggests that the reservoir was deposited in a distal prodeltaic setting, on a storm-dominated shelf. Interpreted sediment transport processes included both hypopycnal and hyperpycnal flows downdip of the delta. Much of the sediment was subsequently reworked by shelf currents.

Early Aptian sandstones are mainly immature ferroan calcite-cemented lithic arkoses, with moderate intergranular porosity (arithmetic mean = 16%). Horizontal permeability is low, with a geometric mean of approximately 1 mD. Cements that reduce porosity to less than 10% are generally pervasive in unburrowed sandstones. Cements are less extensive in bioturbated sandstones, indicating that burrowing may have helped preserve original porosity by inhibiting cementation through physical and chemical mechanisms.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.