本研究利用新竹至台中地區過去40年的井下鑽井資料來推估地下的現地應力狀態。在應力大小方面，區域性鉛直應力(Sv)是利用鐵砧山(TCS)和出磺坑(CHK)地區的密度電測(Density log)積分而得之平均值，其梯度約為23 MPa/km；砂岩層的地層孔隙液壓(Pp) 是由重複地層試驗(Repeat formation test, RFT)中的井底靜壓資料獲得，這些井涵蓋青草湖(CTH)、寶山(PS)、錦水(CS)、出磺坑及鐵砧山等區域。頁岩的孔隙液壓背景值，則可利用聲波電測(Sonic log, SL)的隨深度的聲波走時來計算；獲得上述砂、頁岩地層的孔隙液壓值後，即可判斷出超正常地層孔隙液壓帶的頂部位置(Top of overpressure)以及抬昇侵蝕量。由各區的超正常液壓層頂部位置分佈的深度及所在地層推論竹苗地區的構造最高區為出磺坑背斜，其抬昇侵蝕量亦為最大。水力破壞資料包含在裸孔中施作的滲漏試驗(Leak-off test, LOT)及套管內穿孔施作的泥漿擠注試驗(Fluid injection)和迷你液裂(Mini-frac)，得到的地層破裂壓力可用來推估最小水平應力(Shmin)，其梯度約為17-19 MPa/km。最大水平應力(SHmax)大小是由安德生的臨界應力斷層摩擦理論推求，並假設摩擦係數(μ)為0.6，其梯度約為26-35 MPa/km。由以上測量及推算所得到的各應力值指示出竹苗地區主要的應力場為走向滑移斷層的應力機制；而台中地區靠近車籠埔斷層北段的區域在九二一地震前為逆斷層機制，地震後則轉變為走向滑移斷層的應力場。現地應力的方位，參照World Stress Map (WSM)對井孔崩落(Borehole breakout)的定義，SHmax的方位可以透過四臂井徑電測(four-arm caliper tools)來推估。本研究共使用8口鑽井計算SHmax主要的方位為西北－東南走向，但細部研究此一方向會受地形及構造的影響而改變；甚至井深、鑽遇地層及構造(如斷層通過)也會影響主應力方位。在淺部不論是應力大小或方位，都呈現缺乏有系統的變化；應力值會在走向滑移斷層與正斷層應力場之間轉換，同時應力方位也會隨之而變。This study uses subsurface geological data from wells drilled in the past 40 years to estimate the in-situ stress from Hsinchu to Taichung area. The gradient of vertical stress (Sv) estimated from the formation density log data in Tiehchenshan (TCS) and Chuhuangkeng (CHK) region is about 23 MPa/km. From the repeat formation tests (RFT) collected in Chingtsaohu (CTH), Paoshan (PS), Chinshui (CS), Chuhuangkeng and Tiehchenshan regions, pore pressure (Pp) in sandstone of individual structure can be obtained. Additional background pore pressure values in shale can be estimated from wireline Sonic logs. The top of over-pressure as well as amount of erosional uplift can be inferred based in the plot of pore pressures vs. depth. The Chuhuankeng anticline is highest structure in the Hsinchu and Miaoli area in terms of elevater of the over-pressure zone. Hydraulic fracturing data including leak-off tests (LOTs) and fluid injection in cased and cemented wellbores (mini-fracs) in study area provide a good estimate the minimum horizontal stress (Shmin), which has a gradient about 17~19 MPa/km. The magnitude of maximum horizontal stress (SHmax) was constrained by frictional limits from Anderson’s theory of faulting and by presence of drilling induced tensile fractures. The gradient of the SHmax is about 26~35 MPa/km assuming frictional coefficient (μ) of 0.6. The measured In-situ stress magnitudes indicate a predominantly strike-slip stress regime in Hsinchu and Miaoli area. Applying the methodology of judging borehole breakouts in World Stress Map (WMS), orientations of maximum horizontal stresses were calculated from four-arm caliper tools surveyed in 8 wells. The maximum horizontal stress is predominantly orientated in NW-SE. Detailed SHmax orientation will be affected by surface topography and subsurface structures that wells encountered. The magnitude or the orientation of the in-situ stresses vary at shallow depths. The stress state flips from strike-slip and normal fault, and the SHmax (or Shmin) orientations can also change accordingly.
