堆積盆地に発達する頁岩スミアの連続性と流体移動特性

To quantify the transport properties of faults, a series of laboratory experiments is performed on permeability change during deformation of simulated shale smear. The fluid sealing potential of faults is important for better understanding of the hydrocarbon migrationaccumulation process and abnormal fluid pressure build-up below faults in a sedimentary basin. One of the possible mechanisms of the fault sealing process is role of shale smear characterized by migration of mudstone into the fault plane, forming a barrier to the fluid flow. To evaluate the sealing potential of a fault, it is necessary to develop tools and methodology to monitor the permeability of faults during deformation, and to compare experimental results with field observations. First, the author investigated the smear continuity developed in deltaic sediments on Airport Road outcrop, Miri, Sarawak, Malaysia. At this site, the critical shale smear factor (SSF_crt : showing maximum continuous smear at a shale-thickness/throw data) was approximately 8, which is larger than previously reported data at other sites. Then the author conducted experiments using sandstone-shale interbedded specimens obtained from the Miri formation. The natural specimen was inserted into a part of an artificial fault of pre-cut Berea sandstone, and the permeability of the sample was measured. Similar to results of the previous experiments on siltstone by Takahashi (2003), permeability show the following three stages : (1) Regime 1, rapid reduction due to compaction prior to faulting, (2) Regime 2, constant and relatively lower permeability, and (3) Regime 3, permeability recovery caused by erosion of smear. Except for an experiment with a sandy specimen, moreover, all experiments show permeability recovery of 0.5 orders of magnitude after Regime 1. This characteristic permeability change from Regime 1 to Regime 2 for mud-rich specimens appears to indicate dilation induced by fault propagation in the over-consolidated specimen at starting fault sliding. Fault structures observed from the outcrop and from experimental products are similar. The laboratory experiments can thus accurately reproduce the mechanism of fault sealing by shale smear, suggesting that experimental studies are essential to understand fault sealing by shale smear.