Fractal study on fault system of Carboniferous in Junggar Basin based on GIS
Fault system is a significant evidence of tectonic movement during crust tectonic evolution and may play an more important role in oil-gas accumulation process than other tectonic types in sedimentary basin. Carboniferous surface faults in Junggar Basin developed well and varied in size and distribution. There are about 200 faults in Carboniferous, and 187 of them are thrust faults. Chaos-fractals theories have been widely investigated and great progress has been made in the past three decades. One of the important conception-fractal dimension had become a powerful tool for describing non-linearity dynamical system characteristic. The clustered objects in nature are often fractal and fault system distribution in space is inhomogeneous, always occurs in groups, so we can describe spatial distribution of faults from the point of fractal dimension. Fractal dimension of fault system is a comprehensive factor associated with fault number, size, combination modes and dynamics mechanism, so it can evaluate the complexity of fault system quantitatively. The relationship between fault system and oil-gas accumulation is a focus and difficulty problem in petroleum geology, and fractal dimension is a new tool for describing fault distribution and predicting potential areas of hydrocarbon resources. Geographic Information System (GIS) is a kind of technological system collecting, storing, managing, computing, analyzing, displaying and describing the geospatial information supported by computer software and hardware. In the last 15–20 years, GIS have been increasingly used to address a wide variety of geoscience problems. Weights-of-evidence models use the theory of conditional probability to quantify spatial association between fractal dimension and oil-gas accumulation. The weights of evidence are combined with the prior probability of occurrence of oil-gas accumulation using Bayes'rule in a loglinear form under an assumption of conditional independence of the dimension maps to derive posterior probability of occurrence of oil-gas accumulation. In this paper, we first vectorize the fault system in Carboniferous of Junggar Basin in GIS software and store it as polyline layer in Geodatabase of GIS to manage and analyze, then calculate the fractal dimension of three types which are box dimension, information dimension and cumulative length dimension using spatial functions of GIS, in the last use weights-of-evidence model to calculate the correlation coefficients in GIS environment between oil-gas accumulation and three types of fractal dimension in order to quantity the importance of fault system.
Quantitative fracture prediction from seismic data
ABSTRACT This paper presents results obtained from an area located east of Bremen, Germany, where gas is produced from a deep Rotliegend sandstone reservoir. Faults, fractures and associated deformation bands at reservoir depth have an important influence on the productivity of the gas field as fractures are cemented and tight and may act as permeability barriers. This contribution comprises the development of new coherency tools to better image sub-seismic faults and lineaments from seismic data, and the development of fracture attributes in order to quantify fracturation and its areal distribution. The fractal behaviour of faults was used to establish a relationship between coherency processed seismic data and borehole images at log scale. The ‘fractal dimension’ (FD) of the length of a fault population can be interpreted as a characteristic parameter describing local geology in terms of fracturation. Calculating FD for each point of a seismic grid yields an areal distribution of this value. Correlating seismic-derived FD values and fracture populations derived from borehole images defined a linear relationship which can be used to forecast the distribution of sub-seismic fractures and deformation from seismic data.
Caledonian deformation upon southwest Baltica and its tectonic implications: Alternatives and consequences
The continuation of the basement of the southwestern Baltic Shield toward the SW is of major interest for the Caledonian evolution in the southwestern Baltic Sea and northern Germany. A reinterpretation of various marine seismic data suggests a reconsideration of the Caledonian collision pattern. The Sorgenfrei Tornquist Zone forming the northwestern extension of the Tornquist Zone is regarded as the northeasterly boundary fault and the Elbe Lineament is regarded as the southwestern limit of an area which suffered from the docking of East Avalonia to Baltica. It shows many northwestsoutheast striking structures, anomalies, and fault patterns. From our studies, two alternative models emerge: one which considers the Caledonian Deformation Front as the northeastern limit of East Avalonia and a zone of major transpression, collision, and subduction of the Tornquist Sea. Another model considers the Elbe Lineament in Schleswig-Holstein (NW Germany) as a major (dextral) strike-slip fault which is interpreted as the northeasterly limit of East Avalonia and the southwest boundary of the Baltic Shield. Along the Elbe Lineament the Tornquist Sea plus rotating Avalonia were displaced to the northwest. A large part of the area to be studied might have developed into a passive margin toward the Upper Ordovician. A decision which of the two models is to be prefered has to be the major goal of future geophysical experiments.