The Impact of Fault Zone on Hydrocarbon Migration and Accumulation in Faulted Basin

Abstract

Fault and its internal structure have great influences on hydrocarbon migration and accumulation in faulted basins, and the mechanism of hydrocarbon migration along fault zones is a difficulty in petroleum geology. Several fault asphalt zones were found in Dongying Depression, Bohai Bay Basin, which implies that they were once the place for huge hydrocarbon migration and accumulation, and had a firm connection with fault cavities. Larger growth-faults are capable to form bigger cavities along fault zone due to differential deformation and displacement of the two plates during the active faulting period. The characteristics of fault cavities mainly depend upon stress state, lithology, and the shape of fault plane. Areas of high irregularity of the faullt plane being more favorable for the occurrence of fault cavities. The fault cavities act as transfer stations of hydrocarbon migration. When the fault is active, fault cavities will be preferential migration channels and superior accumulation space due to temporary low pressure, and once hydrocarbon migrates into the cavities, light components leak firstly, leaving the remains of heavy residue such as asphalts. According to previous study and outcrop observations, fault zones can be divided into two structural units which are damage zone (sliding breaking zone) and induced fracture zone. The two units are obviously different in mircro-structure, filling material and physical property which leads to the difference in logging response and transporting ability. Using parallel comparison of single well and crosswise comparison between wells, the influences of lithology on logging data can be removed when distinguishing the fault zone from logging data. A well drilling into fault zone and a nearby well out of the fault zone were chosen to carry out the parallel comparison in this study. The two wells have similar sedimentary environment and post-deposition process at the same depth, indicating they should have the same logging response primarily. Thus, the differences in logging response can be used to identify fault zones. Single well crosswise comparison is to compare induced fracture zone, damage zone and original rock zone of a single well drilling into fault zone to highlight the fault zone in logging response. The study shows that the damage zone has the characteristics of low AC (Sonic Logging), low CNL, high DEN while the induced fracture zone has the characteristics of high AC, high CNL and low DEN. Quantitative analyses are conducted on internal structures to classify internal structures of faults by means of instruction curves and intersection mapping. On the base of the structure of the hanging wall and underlying wall plates, the fault zone can be divided into four types, which are complete type (with damage zone and induced fracture zone in both plates), incomplete I type (with either damage zone or induced fracture zone in both plates), incomplete II type (only the hanging wall with damage zone or induced fracture zone), and complex type (very difficult to identify the fault zone structure), and the complete type is the most common one. The analysis of fault structure, fault-sand matching, and the fluid potential shows that oil migrated preferentially to the updip sand along the fault, and tends to form backward fault sealing reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015