Geochemical identification of marine and terrigenous condensates—A case study from the Sichuan Basin, SW China

Light hydrocarbon has abundant geochemical information, but there are few studies on it in Shenmu gas field. Taking Upper Paleozoic in Shenmu gas field as an example, authors use gas chromatography technology to study light hydrocarbon systematically. The results show that (1) The Shenmu gas field is mainly coal-derived gas, which is mixed by partial oil-derived gas due to the experiment data. (2) Based on K 1 , K 2 parameter and Halpern star chart, the Upper Paleozoic gas in Shenmu gas field belongs to the same petroleum system and the depositional environment of natural gas source rocks should be homologous. (3) The source rocks are mainly from terrestrial higher plant origins and belong to swamp facies humic due to methyl cyclohexane index and Mango parameter intersection chart, which excluded the possibility of the Upper Paleozoic limestone as source rocks. (4) The isoheptane ranges from 1.45 to 2.69 with an average of 2.32, and n-heptane ranges from 9.48 to 17.68% with an average of 11.71%, which is below 20%. The maturity of Upper Paleozoic gas in Shenmu gas field is low-normal stage, which is consistent with R o data. (5) The Upper Paleozoic natural gas in the Shenmu gas field did not experience prolonged migration or secondary changes, thus can be analyzed by light hydrocarbon index precisely.

Section: Natural Gas Typesmentioning

confidence: 98%

Light hydrocarbon geochemical characteristics and their application in Upper Paleozoic, Shenmu gas field, Ordos Basin

Han

Tao

et al. 2016

“…The lithologies of the Xu1 Member ( T 3 x 1 ), Xu3 Member ( T 3 x 3 ) and Xu5 Member ( T 3 x 5 ) are mainly black shale, and mudstone with coal seams. These members are the regional petroleum source rocks and seal units of the Xujiahe Formation (Huang et al., 2014; Zhang et al., 2016).…”

Section: Geological Settingmentioning

confidence: 99%

Diagenesis and reservoir characteristics analysis of the Late Triassic Xujiahe Formation tight gas sandstone in the northern Sichuan Basin, China

Lin

Zhai

et al. 2017

The Late Triassic Xujiahe Formation is a key target for tight gas in the northern Sichuan Basin. Thin section, scanning electron microscopy, X-ray diffraction, porosity and permeability analyses have been performed to delineate the diagenesis and reservoir characteristics of Xujiahe sandstone. The results show that the Xujiahe Formation contains feldspathic litharenite, litharenite, sublitharenite and quartzarenite sandstone. Sandstones of the Xujiahe Formation are characterized by low feldspar content and both secondary and micro-fracture porosity. Porosity and permeability analyses of 185 core samples show a broad but low porosity range from 0.79 to 10.43% (average 4.55%) and wide but low permeability range (0.0021-26.001 mD, average 0.449 mD). The higher permeabilities result from micro-fracturing. Strong mechanical compaction plays a more important role in reducing primary porosity of sandstone than cementation during eodiagenesis. Carbonate cement is detrimental to reservoir porosity. Early carbonate cement precipitated from depositional water during eodiagenesis can block primary pores while late carbonate cement formed during mesodiagenesis can fill secondary pores. Quartz cement shows a slight relationship with porosity and permeability. There is a positive relationship between grain-coating chlorite and porosity and permeability. The effect of diagenesis on the reservoir quality of Xujiahe tight gas sandstone is greater that depositional environment during deep burial.

“…Most of the C 2 /C 3 ratios are higher than 5.5, with an average of 7, and the C 1 /(C 2 +C 3 ) ratio is higher than 20 (Li et al, 2003;Dai et al, 2005b;Hu et al, 2007). The data mentioned above show that the shale gas and dissolved gas in the Yanchang Formation are different from the coal-type gas of the Upper Palaeozoic because they are neither biogenic gases nor pyrolysis gases in a high mature stage, which present The n-heptane (n-C 7 ), methylcyclohexane (MCC 6 ) and various structures of dimethylcyclopentane (∑DMCC 5 ) of C 7 light hydrocarbons can be applied to study natural gas sources (Hu et al, 1990;Dai, 1992;1993;Huang et al, 2014a). Methyl cyclohexane is primarily derived from lignin, cellulose and alcohols of higher plants.…”

Section: Genesis Types Of Natural Gasmentioning

confidence: 99%

Genesis Types and Sources of Mesozoic Lacustrine Shale Gas in the Southern Ordos Basin, NW China

Wang

Lei

2015

The continental lacustrine-fluvial facies are the main sedimentary environment of the late Triassic Yanchang Formation in the Ordos Basin of China. The 7 th Member and 9 th Member of the Yanchang Formation (Chang 7 and Chang 9 for short) are shale members that contain an abundance of rich organic matter, in which, natural gas has been found. Ascertaining the genetic types and sources of the shale gas is of considerable significance to the evaluation of the exploratory potential and to reduce the exploratory risk in the Yangchang Formation. The analysis of the geochemistry characteristics of the shale gas, shale desorbed gas and dissolved gas of crude oil in the Yanchang Formation, of the study area, has have been performed. The result shows that the three types of gases are dominated by methane and that the dryness is less than 0.92 and ranges from 0.64 to 0.8. The three types of gases in the Triassic Yanchang Formation have similar gas components but are completely different from the gas components in Palaeozoic coal-derived type gas. The δ 13 C 1 values of the shale gas, shale desorbed gas and crude oil dissolved gas vary between -41.6‰ and -51.6‰, and the average value is -47.8‰. The δ 13 C 2 varies between -40.1‰ and -33.6‰, and the δ 13 C 3 is between -38.6‰ and -30.8‰. The shale gas, shale desorbed gas and crude oil dissolved gas are all oil-type pyrolysis gases, which originate from the shale of Chang 7 and Chang 9. The results indicate that all of the gases have components of lighter methane carbon isotopes, which is obviously inconsistent with the low thermal maturity (0.7%≤Ro≤1.3%) of the shale in the study area. The experiments demonstrate that the isotope fractionation during the process of gas adsorption and desorption is the primary cause of the inconsistence.