Geochemical characteristics and genetic types of natural gas in Ordovician and TS 1 well in Tahe Oilfield

(Wuxi Institute of Petroleum Geology, China Petrochemical Petroleum Exploration and Development Research Institute, Wuxi 2 14 15 1)

The natural gas of Ordovician and TS 1 well in Tahe Oilfield is mainly hydrocarbon gas, mainly methane. The natural gas in the east of Tahe Oilfield is dry gas, the natural gas in other blocks is typical wet gas, and the natural gas in TS 1 well is typical dry gas. Ordovician in Tahe Oilfield and TS 1 well natural gas have the same parent material source, and both are typical oil-type gases. Ordovician natural gas is a combination of oil-type gases with different maturity, which is kerogen degradation gas, while TS 1 well natural gas is crude oil cracking gas. Carbon dioxide gas in well TS 1 in Tahe Oilfield is produced by thermal metamorphism of carbonate rocks. The formation of Ordovician natural gas in this oilfield has the characteristics of multi-period continuity, including normal associated gas of crude oil in mature stage, condensate associated gas in high mature stage and high temperature cracked gas in high mature stage. The natural gas maturity of Well TS 1 is generally higher than that of the main area of Tahe Oilfield, but lower than that of Well S 1, Well Akkule S 14 and Well S 18 in the east of Tahe Oilfield.

Isotope inversion; Hydrocarbon isotope; Rare gas isotopes; Genetic types of TS 1 well in Tahe oilfield

Geochemical Characteristics and Genetic Types of Well TS 1 and Ordovician Natural Gas in Tahe Oilfield, Tarim Basin

Wang Jie, Gu Yi

(Wuxi Institute of Petroleum Geology, China Petrochemical Company, Wuxi 2 14 15 1)

Hydrocarbon gas volume accounts for the vast majority in TS 1 well and Ordovician natural gas in Tahe Oilfield, and methane is absolutely dominant. The natural gas in the east of Tahe Oilfield belongs to typical dry gas, while the rest belongs to wet gas. The natural gas in TS 1 well is typical dry gas. Well D in Tahe Oilfield and Well TS 1 have the same source of Ordovician natural gas and belong to typical oil-type gas. Ordovician natural gas in Tahe Oilfield is a complex of natural gas at different maturity stages, which contains kerogen cracking gas. However, the natural gas in TS 1 well is still mixed with the petroleum cracking gas. Carbon dioxide is produced by thermal metamorphism of carbonate rocks in Tahe Oilfield and TS 1 Well. The natural gas generated in different stages of Tahe Oilfield is continuous, including oil associated gas in mature stage, condensate associated gas in relatively high mature stage, and cracked gas higher than mature stage. The maturity of natural gas in TS 1 well is greater than that in the main area of Tahe Oilfield and lower than that in the eastern area of Tahe Oilfield.

Genetic characteristics of isotope anti-hydrocarbon isotope rare gas in TS 1 well in Tahe oilfield

For the genetic discrimination of natural gas, predecessors have done a lot of fruitful work and formed feasible methods and indicators [1 ~ 4]. The genetic type of natural gas in natural gas reservoir depends on the origin of dominant components in natural gas, and the study of natural gas origin and gas source lays the foundation for natural gas resource evaluation and exploration. Predecessors have done a lot of research on the macro oil and gas geochemistry of Tarim Basin and the geochemical characteristics of crude oil in Tahe Oilfield and its surrounding areas, and have achieved a series of research results [5 ~ 8], which are not detailed in this paper due to space limitations. But so far, the systematic research results on the geochemical characteristics and genetic types of natural gas in Tahe Oilfield have not been reported. In this paper, the composition, hydrocarbon isotope, rare gas isotope and light hydrocarbon fingerprint characteristics of TS 1 natural gas in Tahe Oilfield and its periphery and in Tahe deep well are comprehensively analyzed. The CO2 carbon isotope and rare gas isotope analysis of natural gas are applied to Tahe area for the first time, and the geochemical characteristics and genetic types of Ordovician and deep natural gas in this area are systematically studied, which provides theoretical basis for the research and exploration of deep natural gas.

Geological background of 1

Tahe Oilfield is located in the southwest slope of Akkule Uplift of Shaya Uplift in Tarim Basin, with Halahatang sag in the west, Caohu sag in the east, Yakela fault uplift in the north, Shuntuogole uplift and Maingard sag in the south, with an area of about 750km2. Akkule Uplift is an ancient uplift developed for a long time on the metamorphic basement of the Pre-Sinian system, which has experienced many times of tectonic movement and deformation superposition. It experienced Caledonian, Hercynian, Indosinian-Yanshan and Himalayan tectonic movements. Due to long-term uplift and weathering and erosion, the middle-upper Ordovician and Silurian-middle-lower Devonian were missing in most areas of uplift, and the lower Ordovician was also eroded to varying degrees. Tahe Oilfield is by far the largest marine hydrocarbon source oilfield in China. Exploration shows that the oil-bearing horizons in Tahe Oilfield are Triassic, Carboniferous and Ordovician, which are mainly concentrated in Ordovician, especially in carbonate rocks of middle and lower Ordovician. Karst fractured-vuggy reservoirs are the most developed and the most important pay zones.

Akkule Uplift formed in the middle and late eastern Hugary, and was uplifted by regional compression in the early Hercynian, forming a big nose protruding in the northeast direction. After the late Hercynian movement and Indosinian-Himalayan movement, it is a favorable direction for oil and gas migration. In order to accelerate the pace of oil and gas exploration in Tahe Oilfield, further explore the reservoir development characteristics and oil and gas distribution law of Lower Paleozoic, and realize the oil and gas exploration goal of "looking for Tahe in Tahe Oilfield", the Jianlong trap on the edge of Cambrian carbonate platform was selected and TS 1 well was deployed. Well TS 1 is located in Area 2 of Tahe Oilfield, with a drilling depth of 8,408 meters. After the exposure of Stegosaurus 1, the drilling was completed, and the drilling horizon was the Lower Churitage Formation of Upper Cambrian (). By analyzing the drilling results of TS 1 well, the deep source, reservoir, caprock and hydrocarbon accumulation conditions in Tahe Oilfield are revealed.

2 Natural gas composition characteristics

Ordovician natural gas in Tahe Oilfield mainly appears in the form of dissolved gas, associated gas or condensate gas. Natural gas components include hydrocarbon gas, CO2, N2 and H2S. Natural gas is mainly composed of hydrocarbon gas, accounting for 89.2% ~ 98.6% of the total gas volume, with an average of 95.2%; Non-hydrocarbon gases are mainly CO2 and N2, with a small amount of H2S. Methane is the main hydrocarbon gas, and its content ranges from 49.8% to 93.6%, with an average of 75.7%. The heavy hydrocarbon content of most natural gas is relatively high, accounting for 3.8% ~ 39.4%, with an average of 18.6%. The drying coefficient (c1∑ c) of natural gas in Tahe Oilfield is between 0.56 and 0.96, with an average value of 0.80, which generally belongs to typical moisture. The natural gas drying coefficients of Well S 14 and Well S60 in Akkule area around Tahe Oilfield are 0.96 and 0.93, respectively, which belong to the dry gas type with high thermal evolution. In addition, according to the carbon isotope composition of methane, the natural gas in the east 1 area of Tahe Oilfield is also a typical dry gas. The drying coefficient of other natural gas is between 0.56 and 0.89, which belongs to typical moisture. In the Ordovician natural gas in Tahe Oilfield, the non-hydrocarbon gases are N2, CO2 and H2S, and the content of non-hydrocarbon gases is not high, which is 1.38%- 10.83%, of which N2 content is 0-8.9%, CO2 content is 0.09%-8.39%, and H2S content is widely distributed, from H2S-free to H2S. The area around Well T740, Well T75 1 and Well T738 in the west of the oilfield has high hydrogen sulfide content, ranging from 20.2- 108.8g/m3, which belongs to the middle and high sulfur-bearing gas area. Outside this area, only the areas around Well T804(K) and Well S9 1 are natural gas containing hydrogen sulfide, and the rest areas rapidly change from middle-high sulfur-bearing areas to low-sulfur-bearing areas, and the hydrogen sulfide content of other wells adjacent to the middle-high sulfur-bearing areas rapidly decays below 1g/m3, which is low-hydrogen sulfide natural gas.

Well TS 1 was tested in the Upper Cambrian Qiulitage Formation () at a depth of 7358m, and a small amount of natural gas was produced. It can be seen from the table 1 that two natural gas samples were collected in this section, and the natural gas components were mainly hydrocarbon gas, accounting for 96.9% and 97.0% of the total volume respectively, among which methane was the absolute dominant natural gas component, with the contents of 94. 1% and 93.9% respectively. The content of heavy hydrocarbons in natural gas is 2.79% and 3. 1 1% respectively. The non-hydrocarbon gas contains a certain amount of N2 and CO2, and the N2 content in the two samples is 2.8 1% and 2.86% respectively, while the CO2 content is very low, which is 0.22% and 0.2 1% respectively. The natural gas drying coefficient of TS 1 well is 0.97, which belongs to typical high heat evolution dry gas.

Table 1ts 1 Gas Composition and Geochemical Characteristics of Well

According to the composition characteristics of Ordovician and TS 1 well natural gas in Tahe Oilfield, both of them are mainly hydrocarbon gas, of which methane is absolutely dominant. The methane content of TS 1 well is higher than all Ordovician natural gas in Tahe Oilfield, and the heavy hydrocarbon content is lower than Ordovician natural gas in Tahe Oilfield. In addition, the natural gas in the east 1 block of this oilfield is typical dry gas, while the natural gas in other blocks is typical wet gas. The natural gas drying coefficient of well TS 1 is 0.97, indicating that the natural gas maturity of well TS 1 is higher than that of Ordovician in Tahe Oilfield, and they are in different evolution stages.

Isotopic composition of carbon, hydrogen and rare gases in natural gas

3. Carbon isotopic composition of1methane

Many scholars have different understandings of the boundary value of inorganic and organic methane carbon isotope δ 13C 1. In this paper, -30‰ is used as the boundary value for dividing inorganic methane and organic methane [13c 1]. The carbon isotope of methane in Ordovician natural gas in Tahe Oilfield is light, ranging from-50.8 ‰ to-30.2 ‰, mainly ranging from-42.9 ‰ to-38.4 ‰, with an average of -40.5‰. δ 13C 1 of natural gas in this area is less than -30‰. Combined with other indicators, the Ordovician natural gas methane in Tahe Oilfield is judged to be organic.

As can be seen from Figure 1, in general, natural gas C1∑ C and C 1/C2 have obvious positive correlation with δ 13C 1, that is, with the increase of drying coefficient and C 1/C2, δ/.

The carbon isotope values of methane in TS 1 well are -38.6‰ and -37.2‰ respectively, which are generally heavier than Ordovician natural gas in Tahe Oilfield, indicating that the thermal evolution degree of TS 1 well natural gas is higher than Ordovician natural gas in Tahe Oilfield.

3.2 Carbon Isotopic Composition of Heavy Hydrocarbon Gas

The carbon isotope series of alkane gas refers to the carbon isotope distribution characteristics of alkane gas molecular carbon number order. Organic alkane gas means that the carbon isotope value of alkane gas molecules increases with the increase of carbon number, which is called positive carbon isotope series, that is, δ 13c 1