Outcrop Analogue Study of Deformation Bands and Their Impact on Rock Properties of the Productive Series in South Caspian Basin

Abstract

Abstract South Caspian basin (Azerbaijan) is the principal place of hydrocarbon accumulation in Productive Series sandstone reservoirs. The basin was complicated by total sub-meridional contraction as a result of north-east compressional deformation which caused generation of deformation bands in various rock types of plunging anticlines with significant influence regarding rock properties. The objective of the study is quantitative estimation of deformation bands impact on rock properties based on field measurements held in Yasamal Valley outcrops in northern-west part of the South Caspian basin and integration of acquired data to analogue steeply dipping reservoir rocks of South Caspian basin.

Geological factors as lithological, structural and petrophysical elements are controlling deterioration or potential enhancement of the rock properties in the South Caspian basin with high concentration of deformation bands. Natural gamma radioactivity, permeability, dips and strikes measurements across wide range of facies were integrated for understanding the impact of these factors. Outcrop plugs with and without deformation bands were tested for Routine and Special Core Analyses, petrographical description, SEM, XRD and CT scan tests to estimate the rock properties at the pore scale.

Field gamma ray measurements analysis reveal that no deformation bands are expected in the rocks with shale volume higher than 32%. Furthermore, probability function of deformation bands occurrence has exponentially declining nature. Higher concentration of deformation bands was observed in the northern part of the Yasamal Valley, where larger volume of iron- bearing calcite-cement precipitation during burial diagenesis has taken place and caused reduction in porosity and permeability. These calcite cements were precipitated in channelized sandstone deposition system and most of them are localized in tabular concretions which generally follow the bedding but, in some cases, terminate within a bed across facies. High amount of calcite cement caused greater impact of deformation bands. The intensity of rock degradation is dictated by mineralogical composition and texture properties. In comparison to the host rock plugs taken from the outcrop, adjacent samples with a single deformation band show 33% and 3% decrease in permeability and porosity, respectively. Dip and dip gradient in sandstone beds do not reveal an obvious impact on deformation bands distribution.

Large volume of calcite cement precipitation was observed in Balakhany reservoirs in north-east limb of the Yasamal Valley which caused greater impact deformation bands and reduction in permeability. Isotopic analysis is recommended to be performed for interpretation of the calcite cement origin and the timing of its precipitation. Impact of deformation bands on permeability can be measured at a plug scale, macro scale estimates would require either whole core flooding or numerical modeling.