Effects of abnormally high heat stress on petroleum in reservoir——An example from the Tazhong 18 Well in the Tarim Basin

An igneous intrusion of 94m thick was discovered intruding into the Silurian sandstone from Tazhong 18 Well. The petroleum previously preserved in the Silurian sandstone reservoir was altered into black carbonaceous bitumen by abnormally high heat stress induced by the igneous intrusion. The reflec-tance of the carbonaceous bitumen reaches as high as 3.54%, indicating that the bitumen had evolved into a high thermal evolution level. Similar to the Silurian samples from the neighboring Tazhong 11, Tazhong 12, Tazhong 45 and Tazhong 47 wells, the distribution of C27, C28 and C29 steranes of the car-bonaceous bitumen is still "V" -shaped and can still be employed as an efficient parameter in oil source correlation. The "V" -shaped distribution indicates that the hydrocarbons from the Tazhong 18 and the neighboring wells were all generated from the Middle-Upper Ordovician hydrocarbon source rocks. However, the oil source correlation parameters associated with and terpanes had been changed greatly by the high heat stress and can no longer be used in oil source correlation. The δ 13C values of the pe-troleum from the neighboring wells are between -32.53‰ and -33.37‰, coincident with those of the Paleozoic marine petroleum in the Tarim Basin. However, the δ 13C values of the carbonaceous bitumen from the Tazhong 18 Well are between -27.18‰ and -29.26‰, isotopically much heavier than the pe-troleum from the neighboring wells. The content of light hydrocarbons (nC14-nC20) of the saturated hydrocarbon fraction in the carbonaceous bitumen is extremely higher than the content of heavy hy-drocarbons. The light/heavy hydrocarbon ratios (ΣnC21-/ΣnC22 ) are between 4.56 and 39.17. In the saturated fraction, the even numbered hydrocarbons are predominant to the odd numbered, and the OEP (Odd to Even Predominance) values are between 0.22 and 0.49. However, the content of light hy-drocarbons in the petroleum from the neighboring wells is relatively low and the content of the even numbered hydrocarbons is almost equal to that of the odd numbered. Compared with the samples from the neighboring wells, the abundance of non-alkylated aromatic hydrocarbons, such as phenanthrenes, and polycyclic aromatic hydrocarbons (PAHs), such as fluoranthane, pyrene, benzoa]anthracene and benzofluoranthene, are relatively high.