This study was performed in order to characterize factors that have decreased the quality of sandstone reservoirs. Various analytical methods have been applied to this study, including sedimentology, petrography, petrophysics and regression analysis.
There are around 170 feet of cores for the study. Two primary depositional settings were identified, these being shallow-marine tidal flat complex and fluvial braided-stream, and there are three main lithofacies of sandstone defined, i.e. laminated sandstone, crossed-bedded sandstone and massive sandstone. These lithofacies exhibit similar petrophysical attributes, with core porosity generally being high ranging from 15-20%, but core permeability being less than 5mD. The sandstones are mineralogically complex as they contain multiple generations of pore-filling authigenic minerals, which are predominantly kaolinite, chlorite and illite, with locally common zeolite and carbonate.
Permeability values of the sandstone reservoirs are strongly controlled by illite as expressed by: k (mD) =131-12.3 *illite, with r >0.50. This finding was supported by SEM analysis that indicates. most illite occurs as authigenic fibers, bridging many pore throats. The relationship between, permeability value and other independent lithological variables in the core could also be best described by: K (mD) = -77.98 .+ 10,.57*(13 + 7.08*kaolinite – 19.55*illite using a geological sense-controlled multivariate regression analysis. The combined SEM and mercury injection analysis verified that there is significant microporosity in most of the reservoir intervals, particularly in laminated sandstone. This liquid-ineffective porosity is closely associated with authigenic kaolinite and chlorite. This core- to pore-level reservoir study has shown that the sandstone reservoirs are more controlled by diagenesis rather than depositional environment. This integrated approach also has proposed a geological model that may be applicable in predicting the reservoir quality of stratigraphically equivalent, uncored intervals in adjacent wells.
Author : Sanggam Hutabarat