柴达木盆地昆北断阶带上盘原油成因与成藏特征

昆北断阶带上盘渐新统下部(E31)、古新统-始新统(E1+2)和基岩勘探均获得重大突破,发现亿吨级整装油田。该区沥青与原油并存,分布复杂,不同井区原油富集程度不同,中间切4井区勘探效果较差。通过原油、沥青与源岩生物标志化合物对比,明确指出昆北原油和沥青成因不同,原油为扎哈泉凹陷渐新统上部(E32)源岩成熟阶段产物,沥青来自渐新统下部(E31)源岩,属于低演化阶段并遭破坏稠化的产物。通过原油性质、储层显微荧光、储层包裹体分析,结合源岩和构造演化特征,确定昆北地区存在2期成藏,成藏时间分别在中新世(N1)早期和上新世(N2)晚期以后,目前储层中的油主要是第2期原油。第1期油主要沿基岩和上覆沉积岩层之间不整合面侧向运移成藏,因切6和切12井区古构造位置较高,成藏规模较大,受破坏形成的沥青规模也较大;第2期油先沿昆北断裂运移到上盘,然后沿不整合面侧向运移、后再沿主要次级断裂运移到目前圈闭中成藏,主要次级断裂对油的运移成藏起重要控制作用。远离切16井东断层、E1+2储层与切163井区不连通是导致目前切4井区勘探效果较差的主要原因。

Origin and accumulation characteristics of the oil from hanging wails of Kunbei fault-terrace belt in Qaidam Basin

A great exploration breakthrough has been made in the bedrock, the lower part of Oligocene(E₃¹) and the Paleocene and Eocene(E₁₊₂) hanging walls of Kunbei fault-terrace belt, where an integrated oilfield with over billion tons of oils has been discovered. With the coexistence and complex distribution of bitumen and crude oils, degrees of crude oil enrichment in this area are different in different well fields, and the exploration effect of the Well Qie-4 area in the middle was relatively poor. The biomarker correlation of source rocks, crude oils and bitumen shows that oils and bitumen in Kunbei have different origins. The crude oil was derived from the upper part of Oligocene(E₃²) mature-stage source rock in Zhahaquan sag, while the bitumen originated from the E₃¹ source rock and it has been destroyed and densified during the low evolutionary stage. Based on analyses of crude oil properties, microscopic fluorescence of reservoirs and reservoir inclusions, and combined with evolutionary features of source rocks and their structures, we confirmed that there are two stages of reservoir-forming in Kunbei area, which are the early Miocene(N₁) and the reservoir-forming stage after the late Pliocene(N₂), respectively. The current oil in the reservoirs mainly came from the second stage. The oil from the first stage mainly migrated laterally along unconformities between bed rocks and overlying sedimentary layers to form reservoirs. Because the paleo-tectonic location of the Qie-6 and Qie-12 well fields was higher than the Qie-4 well field, the scope of reservoir formation in the former was larger while compared with the latter, and its dimension of bitumen formed by later destruction was also huger. The oil from the second stage migrated firstly along the Kunbei faults to hanging walls, then laterally along unconformities and finally along main secondary faults to accumulate in the present traps. Consequently, the main secondary fault is a key factor to control the crude oil migration and accumulation. And the poor exploration effect of the Well Qie-4 area was primarily attributed to its location far away from the east fault of the Qie-16 well and disconnection of the E₁₊₂ reservoir with the Qie-163 well field.