FAULT-RELATED FRACTURE CHARACTERIZATION: A QUANTITATIVE APPROACH IN NATURALLY FRACTURED RESERVOIR CHARACTERIZATION

摘要

Gunung Walat Area, Sukabumi, West Java was chosen for this fault-related fracture study because of the good quality outcrops of Bayah Formation which have been created by mining activity. The principle objective of this study was to obtain empirical and functional relationships between faults and fault-related fracture zones. The methods used in this study include field survey and data acquisition, data sorting, data calculation, statistical analysis and interpretation. Field data acquisition obtained proper fault-related fracture zone attributes, with emphasis on fracture density. By using the Riedel shear model, data were sorted to obtain fault-related fracture data and to identify fractures unrelated to faulting. Data calculation results were plotted in graphical form, analyzed statistically, and interpreted geologically. Fault-related fracture intensity appears to be a function of lithology, distance from faults, fault displacement, total strain, burial depth, and fault type (Nelson, 1985). To meet the objectives of this study, the field survey was designed to find faults with different displacement that cut one or more beds. Two sandstone beds, each cut by three faults, were selected for the study. Bed Ⅰ, with a thickness of 1.1 m is cut by 3 faults that show normal displacement. Bed Ⅱ, with a thickness of 7 m is cut by 3 faults that show strike-slip normal displacement. It was assumed that the intensity of fault-related fractures in each bed is a function of distance from fault and amount of fault displacement only. Results show that fracture intensity in the hangingwall block decrease by bimodal patterns with increasing distance from the faults. These bimodal patterns are obtained using 6th-order polynomial functions. By assuming that zero fault displacement relates to zero fracture intensity, it can be demonstrated that with increasing fault displacement, fracture zone width and fracture intensity in the hangingwall block increase following a logarithmic pattern. Results also indicate that for the same fault displacement, fracture zones in the thicker bed are wider and less intense when compared to fracture zones in the thinner bed. The results of this study can be regarded as a first step toward predicting subsurface fracture system attributes along faults. The results of this study may be useful in predicting: (ⅰ) productive fracture zone width, (ⅱ) geometrical aspects of productive fracture zone, and (ⅲ) fracture porosity and permeability.