Major factors controlling lamina induced fractures in the Upper Triassic Yanchang formation tight oil reservoir, Ordos basin, China

摘要

Lamina induced fractures (LF) are abundant in lacustrine formations, playing an important role for the tight oil resource exploration. Studies of controlling factors of the LF are still lacking, especially for the heterogeneous formations characterized with complex combinations of the lamina and surrounding rock. Therefore, 28 long column samples for triaxial compression tests, 37 short column samples for Brazil split tests and 12 powder samples for mineral component analysis and isotopes analysis (carbon and oxygen) are made from 6 cores in 4 petrophysical faces. The experiments are designed to study the LF propagation process, considering the con- trolling factors in terms of mineral component, the lamina angle and the petrophysical faces. Our results show that: Clay content and dolomite cement degree more significantly control the LF propagation than the other minerals; The LF propagation is not only dependent on the mineral component in the lamina, but even more on the mineral component difference between the lamina and the surrounding rock (i.e., the L/S values), where 'L' represents the lamina and 'S' represents the surrounding rock adjacent to the lamina; The LF tended to propagate in a middle lamina angle range (approximately 10 degrees-60 degrees) in shear properties, and characterized with lower compression strength values; All the LF observed in the thin sections were associated with oil, although there was no obvious oil shown in the study cores; The delta C-13 and delta O-18 values of lamina samples varied with the lamina mineral component for the complex organic matter oxidation and degradation reactions. Additionally, the hydrocarbon flow parameter (P-HF) has been proposed on the carbon and oxidation isotope trace lines, to indicate the oil-rock reactions during the LF open period.