Depositional pattern and reservoir heterogeneity of Tipam Sand-2, Lakwa Field, India

Abstract

This paper describes the importance of the braided channel environment for the entrapment of hydrocarbons, the porosity pattern, the reservoir heterogeneity, the sweep efficiency and the identification of bypassed oil areas, in the Miocene Tipam Sand-2 of the Lakwa Oilfield. The growth-faulted structure has an oil bearing area of 20 km2. The oil gravity varies from 20.7 to 32° API. The Tipam Sand-2 oil pool is the largest among multiple pools in the Tipam Sandstone Formation. There is a marked coincidence of the structure of the field and the thickness of the sand within its depocentre. The sandstone was deposited in a composite braided channel river system. The sand member is characterised by longitudinal and transverse bars which are flanked by marginal, abandoned and slough channel facies. The bars are covered by overbank shales of flood periods. The porosity pattern is dominated by high porosity areas within the longitudinal and transverse bars. The low porosity areas along the flanks of the structure are marked by low energy deposits. Two preferential permeability trends are parallel to the E-W running bar crests, the primary trend, and to the secondary NE-SW or accretionary trend. These areas correlate well with the well production rates, advance of waterfront and areas of better sweep efficiency. The rise in oil-water contact is controlled by reservoir rock permeability trends and oil withdrawal rates. Infill drilling electrolog data indicate a significantly differential rise of the oil-water contact from the initial oil-water contact (5-8 m), as compared to the nearby wells (10-15 m). Integrating the depositional pattern. The reservoir heterogeneity and the withdrawal rates, a reservoir zoning is attempted to identify areas of bypassed mobile oil, especially along the southern part of the structure. Watercut analysis indicates that the preferential paths are along the permeability trends. Water coning is controlled by the presence of the overbank shales, larger clearance of perforations from oil-water contact, presence of shaly sands, laminations of silt, mica and cross-bedded foreset laminae.