Fault Sealing for Hydrocarbon Migration and Entrapment：Concept, Characterization and Effect in Migration

Xiaorong Luo¹, Likuan Zhang¹, Guoqi Song², Fenggui Sui², Guiqiang Qiu², Chengpen Song¹, and Jian Zhao^1
1Key Laboratory of Gas Geochemistry, Institute of Geology & Geophysics, Chinese Academy of Sciences, Beijing, China
²Institute of Exploration and Development, Shengli Oilfield Company, SinoPec, Dongying Shandong, China

Fault plane is, in fact, not a simple surface, but a belt consisting of fault gouge, fault damaged zone, fault deriving fractures and failures. For the aim of hydrocarbon migration, the sealing characteristics during movements of a fault should, therefore, be considered as a geological phenomenon, instead of a physical one, by taking all the sealing/opening activities, that happened in a geological period of hydrocarbon migration and accumulation, into account.

The Wangjiagang fault zones lying on the southern slope of the Dongying Sag in Bohai Bar Basin were selected as studying area. Through statistic multi-analysis, three factors are considered to be critical to fault sealability: pore pressure (P) in shale, shale gouge ratio (SGR) in the vicinity of faults, and normal stress (s) on fault plane. To conclude the integrative effects of these factors, a fault opening coefficient (FOC) is defined as directly proportional to P and inversely to s and SGR. By dividing a fault plane into small zones, the value of FOC can be calculated, and the sealability of fault at one zone (as indicated by sealing probability, Ps) is identified by checking if oil was found or not in the reservoirs over one zone.

Some relationships between FOC and Ps during migration seem to exist in our field case study in the Bohai Bar Basin, China: Ps tends to be 1 when FOC is smaller than 1; a power relationship exists between FOC and Ps when FOC is between 1 and 3.5; and Ps tends to be 0 when FOC is larger than 3.5. The relationships suggest that faults varied its behaviors (open or close) during hydrocarbon migration in different time and locations.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.