柴达木盆地西部第三系盐湖相有效生油岩的识别

Identification of Effective Source Rocks in the Tertiary Evaporate Facies in the Western Qaidam Basin

柴达木盆地西部第三系发现了储量可观的油气资源 ,但是总体上讲这里的生油层钙质含量高、有机质丰度低 ;如何识别有效生油岩 ,正确评价油气资源潜量 ,成为这里油气勘探和地球化学的首要问题。通过上、下干柴沟组生油岩的沉积特征和地球化学分析 ,可发现盐湖相存在许多有机质丰度较高的生油岩 ;利用热解和模拟实验等方法确定出有效生油岩的有机碳含量下限为 0 .4%。这样既为该区生油岩提供了评价标准 ,又找到大量有效生油岩 。

Although there have been large volumes of oil and gas discovered from the Tertiary reservoirs in the western Qaidam basin, the source rocks have been believed as the mudstones and shales with high calcite contents and low organic carbon contents (TOC as low as 0.2%) which were deposited in the Tertiary evaporate facies. The mudstones and shales are in great thick and distributed widely in the western Qaidam basin. In general sense, an effective source rock is referred to the rock with enough TOC, which expels oil when its generating oil is over a saturation in the porosity of the rock at proper buried depths or subsurface temperatures. If a rock with very low TOC, its generating oil could not be over the saturation for oil to be expelled out, the rock was called as ineffective source rock. Most samples analyzed by previous studies belong to the ineffective source rocks as their original TOC too low. Therefore, how to identify the effective source rocks is one of key aspects in petroleum resource assess ment on the Tertiary of the Qaidam basin. By sedimentology study on the potential source rocks (dark colored mudstones and shales in the evaporate facies), the authors of this article discovered that there were significantly vertical changes in lithologies and TOC contents in the mudstones and shales, even in a single rhythmic layer of the rocks. The thickness of one rhythmic layer was mostly less than 1 or 2 meters. The rhythmic layer generally consists of marl, calcareous mudstone or shale, gyps shale, saliferous shale, gypsum and halite from the bottom to the top. The gray colored marls and calcareous shales took a large part of the thickness in the rhythmic layer, but their TOC contents were generally less than 0.4%. The dark gray and black colored shales, gyps shales, and saliferous shales were in small thickness (about 10～50 cm) in one rhythmic layer, but rich in TOC (mostly over 0.8%, some of them over 1%, even over 2%). However, identifying the effective source rocks in such different lithologies becomes a key task in hydrocarbon resource assessment within the evaporate sequence. The authors designed a high pressured thermal simulation and combination with the Rock-Eva pyrolysis on the samples with different TOC contents and different lithologies, and found that the original TOC contents of the effective source rocks were at least over 0.4%. The simulation and result were described in detail in this paper.This paper investigated the effective source rocks mainly on hydrocarbon generating from kerogen and expelling from the shales, but not on immature oil generating and expelling. Therefore, the effective source rocks postulated in this paper do not include the immature effective source rocks. The simulations conducted by the authors were ended corresponding to 0.9%～1.0% of the R o, i.e. at about the peak generation of oil from kerogen. If no oil was expelled from the rock sample at the peak oil generation from kerogen, the rock was known as ineffective oil source rock. But with increase of the thermal maturation, gas could be generated and expelled from the rock sample so that the rock might be called as effective gas source rock. However, the maturity of the Tertiary potential source rocks was not very high, the corresponding R o is generally less than 1%, and little gas was generated from the source rocks. Therefore, 0.4% of TOC is an useful standard for identifying the effective source rocks in the evaporate sequence, and a helpful index to finding the oil and gas in the Tertiary of the Western Qaidam basin. Though the effective source rocks defined in this paper are much thinner and less volume than those described in previous studies, much more oil may expel from the effective source rocks because of their high potential of oil generation than those of bulk volume of the marls and calcareous shales.