DIAGENESIS AND PORE EVOLUTION OF CHANG 6 OIL RESERVOIR IN TIEBIANCHENG AREA, ORDOS BASIN

Diagenesis and micro-pore evolution of low-permeability tight sandstone oil reservoirs are the core research fields for efficient development of tight reservoirs. The Chang 6 Member of the Upper Triassic Yanchang Formation in the Tiebiancheng area of the Jiyuan Oilfield in the Ordos Basin is a typical low-permeability-ultra-low-permeability reservoir of terrigenous clastic deposits with multiple provenances. In this paper, the sedimentology, petrology, and physical properties of the Chang 6 member tight sandstone reservoir in this area have been systematically studied. Furthermore, the diagenesis and pore evolution process of the Chang 6 reservoir have been quantitatively described, using a large number of cores, thin sections, scanning electron microscopy, physical property tests, X-ray diffraction and threedimensional CT scanning experiments. The research results show that the lithologies of Chang 6 reservoir are mainly gray, dark gray, medium to fine sandstone and siltstone. According to statistic results, the porosity distribution interval of Chang 6 reservoir is 0.44%-19.32%, with an average value of 10.86%; the permeability distribution interval of Chang 6 reservoir is 0.021 x 10(-3) mu m(2)similar to 18.45x 10(-3) mu m(2), with an average value of 0.91 x 10(-3) mu m(2). Three-dimensional CT scan results showed that the Chang 6 mamber mainly developed intergranular pores, and the micro-pore throat connectivity is good. There are few isolated pore throats, and the pore throats are evenly distributed. The pore diameters are mostly distributed between 3 similar to 324 mu m, the pore volumes are between 10-500 mu m(3), and the pore length-to-width ratio L/W>3. The porosity of the Chang 6 sample has a good positive correlation with permeability. A large number of casting thin sections and scanning electron microscope observations and statistical results showed that the sandstone reservoir has undergone compaction, cementation, dissolution, rupture and metasomatism. When the rigid component content exceeds 4%, it has good anti-compaction and porosity preservation effects. Kaolinite directly fills the pores, which on the one hand makes the pore space smaller; on the other hand, kaolinite replaces the feldspar particles, turning the space originally belonging to the clastic particles into tiny pores with intercrystalline micropores. At the same time, the ratio of dissolution pores increased by the accompanying dissolution has increased the porosity of the reservoir to a certain extent. The impact of chlorite on the physical properties of the reservoir is divided into two stages. The early edge-lined chlorite contains relatively little content, which plays a constructive role in preserving pores; the late-filled chlorite mostly fills the pores and blocks the throats. Illite is mostly associated with chlorite, carbonate rock, siliceous, etc., and it has a destructive effect on the physical properties of the reservoir. The dissolution of the Chang 6 member is strong, and the feldspar dissolution pores are developed, which increases the ratio of intergranular and intragranular dissolution pores. Comprehensive research showed that the Chang 6 member is in the late middle diagenesis A stage, and part of it has entered the early middle diagenesis B stage.