Integration of petrographic and petrophysical logs analyses to characterize and assess reservoir quality of the lower cretaceous sediments in the Orange basin, offshore south africa

Abstract

Commercial hydrocarbon production relies on porosity and permeability that defines the storage capacity and flow capacity of the resevoir. To assess these parameters, petrographic and petrophysical log analyses has been found as one of the most powerful approach. The approach has become ideal in determining reservoir quality of uncored reservoirs following regression technique. It is upon this background that a need arises to integrate petrographic and petrophysical well data from the study area. Thus, this project gives first hand information about the reservoir quality for hydrocarbon producibility. Five wells (A-J1, A-D1, A-H1, A-K1 and K-A2) were studied within the Orange Basin, Offshore South Africa and thirty five (35) reservoirs were defined on gamma ray log where sandstone thickness is greater than 10m. Eighty three (83) sandstone samples were gathered from these reservoirs for petrographic analyses within Hauterevian to Cenomanian sequences. Thin section analyses of these sediments revealed pore restriction by quartz and feldspar overgrowths and pore filling by siderite, pyrite, kaolinite, illite, chlorite and calcite. These diagenetic minerals occurrence has distructed intergranular pore space to almost no point count porosity in well K-A2 whilst in A-J1, A-D1, A-H1 and A-K1 wells porosity increases at some zones due to secondary porosity. Volume of clay, porosity, permeability, water saturation, storage capacity, flow capacity and hydrocarbon volume were calculated within the pay sand interval. The average volume of clay ranged from 6% to 70.5%. The estimated average effective porosity ranged from 10% to 20%. The average water saturation ranged from 21.7% to 53.4%. Permeability ranged from a negligible value to 411.05mD. Storage capacity ranged from 6.56 scf to 2228.17 scf. Flow capacity ranged from 1.70 mD-ft to 31615.82 mD-ft. Hydrocarbon volume varied from 2397.7 cubic feet to 6215.4 cubic feet. Good to very good reservoir qualities were observed in some zones of well A-J1, A-K1 and A-H1 whereas well A-D1 and K-A2 presented poor qualities.