Predicting Oil Quality from Sidewall Cores Using PFID, TEC, and NIR Analytical Techniques in Sandstone Reservoirs, Rio del Rey, Cameroon

W. Owen BeMent, Robert I. McNeil, Robert G. Lippincott

Tertiary sandstone reservoirs in the Rio del Rey Basin, offshore Cameroon, contain oil and thermal gas that has migrated vertically from deeper buried thermally mature marine shale source rocks. Several shallow reservoirs also contain biogenic gas. Generally, lower gravity oils found in the shallow reservoirs have undergone various degrees of bio-degradation. Deeper accumulations are higher gravity 'primary' oils. The biodegraded oils are characterized by lower gravities, higher acid numbers, higher sulfur contents, and higher viscosities than their non-biodegraded counterparts.

Oil quality (API gravity and acid number) has a significant impact is on Cameroon development economics. It is important to obtain as much geochemical information as possible from the limited volume of oil contained in conventional sidewall samples because borehole conditions often preclude the possibility of running the MDT tool to obtain a fluid sample. An analytical program of P-FID (Pyrolysis Flame Ionization Detection), TEC (Thermal Extraction Chromatography) and NIR (Near Infra-Red spectroscopy) was conducted on a set of 'calibration' oils from the Rio del Rey Basin.

These data were used to develop both empirical and quantitative predictive criteria for estimating crude oil properties of API gravity, acid number, sulfur content, and viscosity. These criteria were used to estimate oil quality on 'well site' frozen sidewall core samples obtained from four oil sands encountered during a two well exploratory program in 1993.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995