The upper Paleocene-lower Eocene clastic reservoirs constitute one of the most important hydrocarbon-producing horizons in the Upper Assam basin of northeast India. These reservoirs are characterized by the presence of (1) normal gravity oil (23-31° API), (2) low to intermediate gravity oil (13-23° API), and (3) gas and light oil/condensate (> 31° API). The sandstone reservoirs show complex wire-line log signatures, which are commonly misleading. The main impediments to understanding the nature of reservoir fluid include (1) distinguishing low gravity, intermediate gravity, and normal gravity oil-bearing zones and (2) resolving the problem of density (RHOB)-neutron porosity (NPHI) crossover for gas-bearing, as well as oil-bearing, reservoirs.

Geochemical analyses of sidewall core extracts by thin layer chromatography with flame ionization detection (TLC-FID) and gas chromatography (GC) provide valuable, cost-effective input for reservoir fluid characterization. The important parameters needed to identify the nature and composition of the reservoir fluid include (1) the bulk composition of the sidewall core extract (i.e., % saturated hydrocarbons, % aromatic hydrocarbons, and % resins + asphaltenes), (2) amount of extract in mg/g of rock, (3) GC fingerprint, and (4) the ratio of pristane to n-C17. These parameters, integrated with geological and geophysical (wire-line logs) evidence, yield more accurate and reliable formation evaluation criteria. The technique is simple and inexpensive and may find application as an additional formation evaluation tool in any geological setting.

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