|dc.description.abstract||The main oil and gas production structure is a series of parallel anticlines in NNE-SSW orientation in western foothills. Chinshui and Chuhuangkeng anticlines are the most productive structures among them. The CPC (Chinese Petroleum Corporation) surmises that Pakuali structure, which is east of Chuhuangkeng anticline, could also have the potential to be a production structure. Nevertheless, it is inefficient to carry out geophysical exploration and field investigation in the inner western foothills where Pakuali structure is located. Taking advantage of the abundant surface and subsurface geological data of anticlines west of the Pakuali structure, it is legitimate to reason the tectonic characteristics and structural evolution of Chuhuangkeng-Pakuali area. The aim of this study is to construct a geological cross-section which can be balanced kinematically and simulated mechanically by MOVE commercial software and PFC2D simulation software, respectively. Furthermore, fracture distribution analysis and porosity assessment are performed based on the simulation results.
Based on my cross-section, the structural characteristics of crustal folds in the western foothills in Miaoli are similar to the folds produced by the mechanism of buckling. Accordingly, the numerical simulation of this study are founded on the buckling mechanism. The stratigraphic assemblage of the oldest Wuchishan Formation to Cholan Formation are collated and divided into eight structural lithic units, re-named as layer A to H, based on the difference of lithology and mechanical properties. According to the theory of multi-layer folding, the geological cross-section is modified as lobate and cuspate shapes for the contacts of the soft and hard layers at the fold axis, and the structural section is then used as a reference for the successful simulation of the Chuhuangkeng anticline and the Pakuali structure.
The results show that when the hard layer strength is 25 times and 50 times greater than the soft layer strength and the horizontal shortening is 38%, the structural configuration of the Pakuali structure and the Chuhuangkeng anticline can be successfully simulated. For the case of layer strength ratio of 25 times, the cracks are concentrated in the western limb of the Chinshui anticline and the western limb of the Pakuali structure. There are also sporadic cracks in the both limb of the Chuhuangkeng anticline. The porosity of the eastern limb of the Chuhuangkeng anticline is higher than that of the western limb, and the porosity of the B layer in the eastern limb is about 4.7 to 4.9%. The porosity of the west limb of the Pakuali structure is higher than that of the east limb, and the porosity of the B layer in the west limb is about 8.8 to 9.6%. For the case of layer strength ratio of 50 times, cracks develop in the western limb of the Pakuali structure, both limbs of the Chuhuangkeng anticline and the cores of two structures at the A layer. The porosity of B layer at the eastern limb of the Chuhuangkeng anticline is about 6.0 to 7.1% while the porosity of B layer at the western limb of Pakuali structure is about 4.8 to 9.6%. Comparing the values of porosity from the numerical simulation with the values of porosity from the tests, the porosity values derived from the two-dimensional simulation are, in general, few percentages less than the porosity values obtained from the core test and the NMR test.||en_US|