六盘山盆地固原凹陷构造-热演化史及油气地质意义

Tectono-thermal evolution history and geological significance of Guyuan Depression in Liupanshan Basin

六盘山盆地下白垩统生储盖组合良好,但至今油气勘探仍未有明显成效,烃源岩成熟度及其构造-热演化史是影响该区油气勘探深入的重要因素。文中利用盆地中部固原凹陷GC1井岩心样品分别开展了下白垩统镜质组反射率(R_(o))和磷灰石裂变径迹(AFT)分析测试。结果表明:马东山组暗色泥岩R_(o)介于0.9%~1.1%,已达到成熟生烃阶段。李洼峡组砂岩AFT年龄为37.4±2.4 Ma,且经历了完全退火,共同指示了固原凹陷下白垩统曾经历了较高的热演化及生烃过程。埋藏史及热演化史模拟,进一步指出固原凹陷下白垩统—距今100 Ma达到最大埋深和热演化阶段,最大地温梯度可达51~60℃/km;晚白垩世以来,经历了快速到缓慢的冷却抬升和剥蚀过程,始新世再次发生埋藏升温,并保持至今。上述认识为该区油气勘探评价提供了科学依据。综合区域地质,认为固原凹陷结构受区域断裂控制,然而凹陷区内部改造相对较弱、生储盖配置良好,推测有较好的油气勘探前景。

The Lower Cretaceous source-reservoir-caprock combinations in the Liupanshan Basin develop well, but oil and gas exploration has no obvious breakthrough. The maturity of the source rock and its tectono-thermal evolution history are the main factors restricting its further hydrocarbon exploration. In this paper, vitrinite reflectance(R_(o)) and apatite fission-track(AFT) were measured respectively for core samples of the Low Cretaceous of the Well GC1 of Guyuan Depression in the central of the basin. The results show obtained average value of R_(o) ranges from 0.9% to 1.1%, indicating that the dark mudstone of the Madongshan Formation has reached maturity. The sandstone AFT age of Liwaxia Formation is 37.4±2.4 Ma and has undergone complete annealing, which together indicating that the Lower Cretaceous Guyuan Depression has experienced a high thermal evolution and hydrocarbon generation process. The buried and thermal evolution history modeling further show that the Lower Cretaceous Guyuan Depression reached the maximum burial depth and thermal evolution stage at-100 Ma, and the maximum geothermal gradient in the Early Cretaceous basin is about 51-60 ℃/km. The basin experienced a rapid to slow cooling, uplift and denudation successively in the Late Cretaceous, then it gradually buried and heated up again at Eocene and keep up to now. The above understanding provides a scientific basis for the evaluation of oil and gas exploration in this area. Combining with the regional geological characteristics of the study area, the structure of the Guyuan depression is considered to be controlled by regional faults. However, the reworking is relatively weak in the internal Guyuan Depression, which have good source-reservoir-cap configuration and prospect for oil and gas exploration.