Maturity Assessment of Severely Biodegraded Marine Oils from the Halahatang Depression in Tarim Basin

Abstract

Seventeen Ordovician oil samples from Halahatang Depression are investigated for their thermal maturation. On the basis of GC analysis of the alkane fraction as well as GC-MS analysis of the demethylated hopanes, it is assumed that the oils are mixtures of severely biodegraded and nonbiodegraded oils. Therefore, their maturation is assessed on the basis of the distribution and abundance of compounds strongly resistant to biodegradation. The P 1 together with the P 2 values of C 7 light hydrocarbons suggest that the late filled nonbiodegraded oils for Halahatang are super mature. Triterpane and sterane isomerization maturation parameters indicate they barely reached the peak oil generation stage, consistent with the results obtained from the equivalent vitrinite reflectance Rc 1 (0.80-0.95%), while the Rc 2 (1.28-1.60%) delineate a high maturity of Halahatang oils. However these conventional triterpane and sterane isomerization maturation parameters gave only general information and a comparatively narrow range of maturity. Correlation between the maturation parameters of Pr/nC 17 and Ph/nC 18 , Ts/(Ts + Tm) and C 29 Ts/(C 29 Ts + C 29 H), MPI 1 and MPI 3 , 4-/1-MDBT and 4, 6-/1,4-DMDBT are rather good, confirming the validity to assess and differentiate the maturity of the severely biodegraded oils examined, with the exception of naphthalene maturation parameters(DNR 1 and TNR 2 ). According to the maturation classification assembled with selected effective maturity parameters, such as the MPI 3 together with the corresponding [P], the thermal maturities of Halahatang oils are moderate, while Ts/(Ts + Tm) together with 4-/1-MDBT demonstrate a moderate-high maturity.

Charging of Ordovician Reservoirs in the Halahatang Depression (Tarim Basin, Nw China) Determined by Oil Geochemistry

“…0.52-0.55, at 0.8 ± 0.1% vitrinite reflectance: R o ); and the C 29 sterane ββ/(αα + ββ) ratio is 0.52 to 0.59 (equilibrium value 0.67-0.71). This suggests that the oils have been generated from organic matter which had barely reached the peak of the oil window (Peters, et al, 2005;Chang et al, 2012).…”

“…Constraints from molecular geochemistry Fifteen of the seventeen oils from Halahatang oilfield investigated show an intact n-alkane series and an evident UCM on the gas chromatograms, and a fullydeveloped series of C 26 -C 34 25-norhopanes on m/z 177 mass fragmentograms for the same oils ( Fig. 5d; Chang et al, 2012), with a value of 0.07-1.70 for the C 29 25-norhopane/C 30 hopane ratio (Table 2).…”

Geochemistry and origin of Ordovician oils in the Rewapu Block of the Halahatang Oilfield (NW China)

“…In the northern part of the Halahatang region (Ha6 Block), the heavy oils contain both a full range of nalkanes (indicating no biodegradation) and 25-norhopane series/baseline ''hump'' in gas chromatograms of saturated hydrocarbons (Chang et al 2012;Li et al 2018), indicating the severely biodegraded oils were mixed with undegraded petroleum. The 25-norhopanes (also called 10-demethyl hopanes or demethylated hopanes) are C 26 -C 34 compounds which have similar chemical structure to those of regular hopanes, except for the absence of the methyl group at the A/B ring junction (Peters et al 2005).…”

“…Biodegradation is a well-documented process that modifies the physical properties and chemical compositions of reservoir oils (Huang et al 2003). Much work has been done on the formation of heavy oil and solid bitumen via biodegradation in the Tarim Basin (Tian et al 2008;Chang et al 2012;Zhu et al 2012;Zhang et al 2014;Cheng et al 2016). Recent research also shows that the physical properties and chemical compositions of oils in this region are jointly controlled by the development of caprocks, the extent of mixing of biodegraded and unbiodegraded oils, and the most recent preservation conditions (Li et al 2018).…”

Geochemical Characterization of Released Bound Biomarkers by Catalytic Hydropyrolysis of Ordovician Source Rock from Tarim Basin, China

“…The Tarim Basin in north-western China is a large inland basin with numerous petroleum source rocks, chiefly Cambrian-Ordovician marine sedimentary rocks of the Palaeozoic petroleum systems Zhang et al, 2000;Ma et al, 2006;Gao et al, 2007;Wang et al, 2008;Chang et al, 2012 (Gao et al, 2007). These shales contribute to a large extent to the hydrocarbon generation in the Lower Palaeozoic within the Tarim Basin.…”