川西坳陷鸭子河地区基于多种古温标的钻井热史恢复

利用镜质体反射率(Ro)、磷灰石裂变径迹(AFT)和伊利石结晶度(IC指数)等古温标恢复了四川盆地川西坳陷的钻井热史,对比了不同温标最高古地温的恢复结果.研究表明,研究区晚白垩世至今总体表现为冷却及抬升剥蚀的过程,地温梯度由约26℃·km-1降低至约22℃·km-1,剥蚀量约1.3~1.9km.约80 Ma以来开始抬升剥蚀,40—2.5 Ma经历了一个热平静期,第四纪存在一定的增温,地温梯度增高约5℃·km-1.三种古地温恢复结果具有较高的一致性,相对于镜质体反射率(Ro)和磷灰石裂变径迹(AFT)等成熟古温标,伊利石结晶度作为有机质成熟度指标和沉积岩古温标的应用处于定性分析阶段,该指标的热演化模型仍需进一步探索.

Thermal history reconstruction based on multiple paleo-thermal records of the Yazihe area,western Sichuan depression

Thermal history is significative to the geodynamics and petroleum geology of a sedimentary basin, for temperature is important for many geodynamic and the hydrocarbon accumulation process, especially the maturation evolution of hydrocarbon source rocks and the history of hydrocarbon generation.In the western Sichuan Basin, a continental depression is superimposed on the Paleozoic craton. The Paleozoic source rocks in this area have experienced a multi-stage and complex structural-thermal evolution and thus require different methods and effective geothermal indicators to reconstruct the thermal history,which will help discover the equivalence between different geothermal indicators and establish a set of geothermal indicators for the thermal history reconstruction of the area. The thermal history of a basin is usually reconstructed using various paleo-temperature indicators, including vitrinite reflectance (Ro), apatite fission track (AFT), (U-Th)/He in apatite and zircon, reflectance of bitumen and vitrinite-like macerals, the hydrogen index, and fluid inclusions. Ro and AFT are the most widely used indicators and the related modeling methods are relatively well established, other indicators such asillitecry stallinity (IC), Raman spectroscopy, and acoustic rock emissions have also been studied and used in the quantitative reconstruction of paleo-temperatures.In this paper, the thermal history of the western Sichuan depression was reconstructed based on three geothermal indicators: Ro, AFT and the IC index, using the paleo-temperature gradient method, Monte Carlo modeling and the estimation of metamorphic temperature stage respectively, and the maximum paleo-temperatures reconstructed based on different indicators were compared. The thermal history reconstruction of CY92 using the paleo-temperature gradient method shows that the maximum paleo-temperature gradient of this borehole was ~26 ℃·km^-1, the maximum paleo-heat flow was ~60 mW·m^-2, both was larger than the present. The thickness of removed sediments on the surface of the unconformity between the Upper Jurassic and the Cenozoic was approximately 1900 m. This indicates that the area has undergone an overall, continuing structural-thermal evolution of cooling, uplifting, and denudation since the rock strata reached the maximum temperatures.AFT thermal history modeling result of CY92-1 reveals that the basin had experienced subsidence between 220 and 140 Ma and a thermal quiet period between 145 and 80 Ma, crustal uplift and erosion occurred in this area between 80 and 40 Ma, but after another thermal quiet period from 40 to ~2.5 Ma, the temperatures of the strata began to rise. The thickness of the removed sediments was estimated to be ~1900 m, which is generally consistent with the Ro analysis results.In borehole CY95, at a depth of ~4080 m, the KI was 0.60 (°Δ2θ), and the maximum paleo-temperature was ~150 ℃; at a depth of ~6000 m, the KI was 0.42(°Δ2θ), and the maximum paleo-temperature was ~200 ℃. The paleo-temperature profile of CY95 indicates a less of ~690 m in the thickness of removed sediments. In conclusion, the structural-thermal evolution in the western Sichuan depression since the late cretaceous to present is denudating and cooling, the geothermal gradient reduced from about 26 ℃·km^-1 to about 22 ℃·km^-1, the erosion thickness is about 1.3~1.9 km. The uplifted and eroded is continuous since ~80 Ma to 40 Ma, a thermal quiet period kept from 40~2.5 Ma, then the geothermal gradient raised about 5 ℃·km^-1 since about 2.5 Ma. There was high consistency between the paleo-temperatures of the western Sichuan depression that were reconstructed based on Ro, AFT, and KI. Thus, these three indicators can be used as paleo-temperature indicators for the thermal history reconstruction of the area. However, relative to Ro and AFT, the illitecry stallinity index is still controversial and needs more research for using it as a maturity and geothermal indicator in the sedimentary rocks.