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

Key words: 
• evaporate facies /
• effective source rocks /
• petroleum resource assessment /
• minimum TOC /
• Qaidam basin

Abstract: Although there have been large volumes of oil and gas discovered from the Tcrtiarv reservoirs in the western Qaidam basin, the source rocks have been believed as the mudstoncs and shalcs with high calcite contents and low organic carbon contents(TOC as low as 0.2%)which were deposited in the Tcrtiarv evaporate facics.The mudstoncs and shalcs are in great thick and distributed widclv 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 he over the saturation for oil to he 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 mcnt on the Tertiary of the Qaidam basin. Bv scdimcntology study on the potential source rocks(dark colored mudstoncs and shalcs in the evaporate facics,the authors of this article discovered that there were significantly vertical changes in lithologics and TOC contents in the mudstones and shalcs, 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 shalcs, gyps shalcs, and salifcrous shalcs were but rich in TOC(mostly over 0.8 0lo,some of them over 1% in small thickness(about 10-50 em) in one rhythmic laver,even over 2%).However~identifying the effective source rocks in such different lithologics becomes a kcv 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 at least TOC contents and different lithologics, and found that the original TOC contents of the effective source rocks were over 0.4 0}o.The simulation and result were described in detail in this paper. This paper investigated the effective source rocks mainly on hydrocarbon generating from kcrogcn and expelling from the shales, but not on immature oil gcncrating 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,,peak generation of oil from kcrogcn.ff no oil was expelled from the rock sample at the peak oil generation;.P.at about the rock was known gelled from the as ineffective oil source rock.But with increase of the thermal maturation gas could he from kcrogcn, the generated and exrock sample so that the rock might he called as effective gas source rock.However the maturity of the Tcrnary potential source rocks was not very high the corresponding R, 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 cvaporate sequence and a helpful index to finding the oil and gas in the Tertiary of the western Qaidam basin. Though the cffcctivc 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 hulk volume of the marls and calcareous shales.~