Abstract: Gas Behavior in the Campos Basin: a New Approach in Exploration

Mello, M. R.; Takaki, T. - Petrobras/Cenpes; and Prinzhofer, A. - IFP

Gas signatures are used to characterize processes occurring since the HC generation in the source rocks to their accumulations in the reservoirs. Emphasis is put here on two main geological parameters: the maturity of the gas samples, and their degree of segregation during their migration at long distance.

Four new diagrams have been used to better constrain some fields of the Campos basin: 1) a d¹³C2- d¹³C3 versus C2lC3 diagram (Fig. 1), monitoring the genesis of HC gases (primary cracking of kerogen, secondary cracking of the oils, or even tertiary cracking of the wetter part of the hydrocarbon gases, retention time in the source rock, Prinzhofer & Huc, 1995; Prinzhofer & Lorant, 1997; Lorant et al., 1998). 2) a C2/C1 versus d¹³C1 diagram, allowing to distinguish bacterial contamination, segregation during transport, and maturity (Fig. 2). 3) a new star diagram ("Gastar diagram"), showing directly the relative degrees of maturity and segregation in a family of gases. 4) a Principal Component Analysis, quantifying the maturity and the migration of gases (both during accumulation or during leakage).

In the Campos basin, some fields show a small maturity range, when others may go from the oil window to overmature gas samples, involving secondary cracking of oils. These last fields, with a high range of maturity, show little segregation due to migration, when the gases with homogeneous maturity exhibit evidences of long migrating distance. This is confirmed by a statistical study, where it is possible to distinguish both maturity and migration trends.

This study establishes a new way of using gas geochemistry for interpreting both gas and oil history in a sedimentary basin (Prinzhofer method). Gas information is no longer a simple fingerprinting of its origin, but a powerful tracer of geological processes. The main practical information obtained from this case study is the possibility of showing evidence of overmature source rocks in some parts of the basin. This is attested by both the maturity measurements and the evidence of modest distances of migration. This has direct impact for deeper exploration of oil and gas.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil