Karachaganak Field - Lower Permian Integrated Reservoir Study

Francesconi, Arrigo ¹; Bigoni, Francesco ¹; Albertini, Cristian ¹; Villa, Federica ¹; Borromeo, Ornella ¹; Luoni, Fabio ¹
(1)Eni S.p.A E&P Division, San Donato Milanese (Milano), Italy.

Karachaganak Field is an isolated carbonate bank consisting of Carboniferous to Lower Permian carbonate deposits. This paper focuses on the reservoir study concerning the Lower Permian pinnacles which represent one of the future field developments. Core data analysis allowed recognizing two main groups of depositional facies: the bioherm made of both in situ boundstone and reworked breccia related respectively to proximal and distal flanks; the inter bioherm bedded deposits grain-dominated. The depositinal facies have been framed into the reservoir geometry as imaged in the 3D seismic showing a steep isolated horseshoe pinnacle, 9x9 km wide, spanning, in thickness, from few to about 900 m. A comparison with outcrops and subsurface analogues provided models for wells correlation and seismic interpretation suggesting as the pinnacle results from the amalgamation of different mounds. A seismic stratigraphic analysis allowed the identification of boundaries subdividing the reservoir section in three depositional sequences: the Asselian showing aggradation followed by progradation; the Sakmarian interpreted as lowstand deposits; the Artinskian showing a continuous aggrading growth with final exposure. The observed diagenetic dissolution of aragonite, enhancing porosity in the uppermost Asselian deposits, were related to meteoric dissolution at the shift from aggradation to progradation while the dolomite, mainly present on pinnacles flanks and improving the original petrophisical characteristics of these facies, was related to seawater geothermal circulation. The reservoir quality analysis, based on core and log data, suggested stratigraphic and lithofacies control with in-situ brittle boundstones affected by the highest permeabilities related to the occurrence of syn-sedimentary microfractures and vugs. The reservoir model, accordingly built, took advantage of dynamic data as Well Test and production history, to optimize the geological model. These data allowed the tuning, by depositional regions and stratigraphic units, of reservoir permeability according to WT K. The unknown size of the amalgamated microfractured bioherms were calibrated according to the observed production pressure trends characterized by an early abrupt decline, related to the initial drainage of the confined permeable microfractured bioherms, followed by a later stabilization, related to the support from the inter bioherms bedded deposits with low flow capacity but high storage capacity.

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