Complex Karst-controlled Reservoirs in Paleozoic Limestone of the CIS, Kazakhstan an Europe; Identification and Prediction for Distribution of the Reservoir Characteristics

Lapointe, Philippe A.¹, Arnaud Meyer² (1) TOTAL E&P, 64018 Pau, France (2) Total Exploration Production, Pau, France

Carbonate karst-controlled reservoirs are economically interesting. Case histories from CIS, Kazakhstan and Europe show that production depends on the knowledge of the nature of the karst and its organization.

Detailed examination of cores cut in CIS and Kazakhstan fields and Europe reveals the karstic nature, meteoric and coastal/island, of the diagenetic processes that affected the reservoir and includes the following: cave fills, locally with shales, fractures enlarged by leaching, morphologies with erosion overprint and, breccia. These karsts appear polycyclical, and of wide extension, but have a low impact on the overall porosity.

The main reservoir enhancement is related to hydrothermal overprint. The hydrothermal karst follows the faults and fracture corridor, using the polycyclic meteoric karst and the remnants of the coastal mixing water karst as well as stylolite plans, bedding and joints. Open conduits and caverns, geodic sparite cement and other exotic minerals characterize this phase. A low hydrothermal activity episode corresponds to pulverized zones of the limestone, and creates high permeability drains.

Mapping of the actual reservoir remains critical for predicting the best poroperm zone distribution and the prognosis of future production well locations, particularly for horizontal or highly deviated wells. The 3-D approach leads to horizontally-driven and vertically-driven cave developments controlled by all the existing heterogeneities and particularly those related to the tectonic events. The resulting complex reservoir is specific. Its identification and understanding are of paramount importance for reservoir development, particularly in Paleozoic rocks.