鄂尔多斯盆地东南部中生界地层节理发育特征与古应力场

鄂尔多斯盆地东南部中生代地层中发育有六组节理(E-W、N-S、ENE-WSW、NNW-SSE、WNW-ESE、NNE-SSW),并且构成三期的正交节理系统(E-W与N-S、ENE-WSW与NNW-SSE、WNW-ESE与NNE-SSW).三期正交节理系统形成的先后期次为:E-W向和N-S向两组节理最早形成,WNW-ESE向和NNE-SSW向两组节理为第二期形成,ENE-WSW向和NNW-SSE向两组节理则最晚形成.E-W向、N-S向和ENE-WSW向三组节理的节理间距指数(FSI)分析结果表明,节理间距的发育程度除了受岩层厚度控制外,还受区域应力场的控制.E-W向、N-S向和ENE-WSW向三组节理的节理间距率(FSR)值分布范围指示不同组节理在区域上发育程度具有差异性.此外,E-W向的优势节理组的FSR值有超过间距与层厚比值的临界值,而非优势组的SN向节理的FSR值则全部小于临界值,表明E-W向和N-S向两组节理组成最早一期的正交节理系统.盆内中生代地层中的三期正交节理系统,所对应的古应力场分别为:(1)晚侏罗世盆地处在近E-W向的挤压环境下,形成了第一期正交节理系统,为E-W向和N-S向两组节理；应力来源于古太平洋板块向欧亚板块俯冲所产生的NW-SE向的挤压分量.(2)晚白垩世时,来自于古太平洋板块俯冲产生的NW-SE向挤压应力形成了第二期正交节理系统的WNW-ESE向和NNE-SSW向两组节理.(3)晚白垩世末至新生代,印度板块向欧亚板块下的俯冲产生NE-SW向的远程挤压应力,形成第三期正交节理系统的ENE-WSW向和NNW-SSE向两组节理.

Joint development and paleostress field in Mesozoic strata of the southeastern Ordos Basin

Six trends of joint sets (E-W, N-S, ENE-WSW, NNW-SSE, WNW-ESE, NNE-SSW) and three groups of orthogonal joints systems (EW and SN, ENE-WSW and NNW-SSE, NWW-SEE and NNE-SSW) are recognized in the Mesozoic strata within the southeastern Ordos Basin. The orthogonal joint systems developed in three sequences: D1 with E-W and N-S trening joints, D2 with SEE-NWW and NNE-SSW trending joints, and D3 with ENE-WSW and NNW-SSE trending joints. The fracture spacing index (FSI) of three sets of joints (E-W, N-S, ENE-WSW) shows that joint development is controlled by mechanical layer thickness, as well as regional stress. The fracture spacing ratio (FSR) values of the joint sets (E-W, N-S, ENE-WSW) indicate regional variation in joint development. Furthermore, the FSR values of the dominant joint sets (E-W) exceeds the critical ratio of spacing/height, while FSR value of subdominate joint sets (N-S) are less than the critical value, as indicating that the orthogonal joint system of E-W and N-S trending joint sets developed earlier than others. Three sets of orthogonal joints systems correspond to three paleostress fields respectively. (1) The Ordos Basin experienced E-W compression in the Late Jurassic, and the first phase of orthogonal joints systems with the E-W and N-S trending joint sets developed resulting from the far-field effect of NW-directed subduction of the Pacific plate; (2) In the Late Cretaceous, the second phase of orthogonal joints system with WNW-ESE and NNE-SSW trending joint sets developed as a result of the continuing subduction of the Pacific plate; (3) From the Late Cretaceous to Cenozoic, the third phase of orthogonal joints system with ENE-WSW and NNW-SSE trending joint sets developed in responding to the NE-SW compression as a far-field effect of the Indian plate collision to the Eurasian plate.