Abstract: Predicting Gas, Oil, and Water Intervals in the Niger Delta Reservoirs Using Gas Chromatography

D. K. Baskin, R. J. Hwang, K. Purdy

Typically, formation evaluation experts have little difficulty in interpreting wireline log data to assess the type of hydrocarbon present (oil vs. gas) in sand reservoirs. This is usually accomplished by a combination neutron-density tool that detects low hydrogen and low electron densities typical of gas zones, and the Repeat Formation Tester (RFT) which utilize both the pressure gradient and sample acquisition techniques to evaluate reservoir fluid. However, in the Niger Delta, many of the sands exhibit a poor hole conditions often result in stuck tools. In these cases, oil fingerprinting of sidewall core extracts can provide an independent means of identifying the type in the reservoir.

Chromatographic signatures of sidewall core extracts from several Western Delta wells clearly reflect the type of hydrocarbons present in the reservoir. Fingerprints of oil-saturated sands are typical of topped (C_15⁺) whole oils, whereas fingerprints of hydrocarbons extracted from the gas-dominated sands show an abbreviated hydrocarbon distribution more typical of gas condensates. Fingerprints o extracts from water productive intervals show a further abbreviated signature, suggesting that higher molecular-weight hydrocarbons did not migrate into these intervals. These signatures are distinct and intact despite poor sample preservations due to prolonged storage.

Thus, comparisons between chromatograms and RFT determined fluids (oil, gas or water) indicate that chromatography is useful in identifying the type of hydrocarbon present in a reservoir. Moreover, the technique also allows the reevaluation of reservoirs in older wells where sidewall or conventional core samples are available. This technology has been transferred to Chevron operating company since its application potential is worldwide. The Gulf Coast of the U.S., Canada, Indonesia, and Papua New Guinea are possible areas for future application.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994