Note
Migrated hydrocarbons in outcrop samples: revised petroleum
exploration directions in the Tarim Basin
Maowen Li
a,*, Zhongyao Xiao
b, Lloyd Snowdon
a, Renzi Lin
c, Peirong Wang
d,
Dujie Hou
d, Linye Zhang
e, Shuichang Zhang
f, Digang Liang
faGeological Survey of Canada, 3303-33 Street NW Calgary, AB, Canada T2L 2A7 bCNPC Tarim Petroleum Exploration Bureau, Kuerle, Xinjiang, China
cPetroleum University (Beijing), Changping, Beijing 102200, China dJianghan Petroleum University, Jingzhou City, Hubei 434102, China
eSINOPEC Shengli Petroleum Bureau, Dongying, Shandong, China fResearch Institute of Petroleum Exploration and Development, Beijing, China
Received 21 February 2000; accepted 29 March 2000 (returned to author for revision 9 March 2000)
Abstract
The application of age-speci®c biomarker distributions established from mature exploration areas of the Tarim Basin, northwestern China, indicates that most Carboniferous±Permian outcrop samples in the eastern segment of the Southwest Depression, previously believed to have signi®cant petroleum source potential, in fact contain migrated hydrocarbons derived from Cambrian±Lower Ordovician strata. New geochemical results have led to a major revision of petroleum exploration directions in this area.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Dinosteranes; Age speci®c biomarkers; Tarim Basin; Hydrocarbons in outcrop samples
1. Introduction
Sample availability and quality are important factors for a geochemical study to be conducted successfully. This is even more critical in frontier areas where samples of source rocks may not be readily available. In many cases, either no wells have been drilled, or wells were drilled but cored dominantly from reservoir intervals. In order to minimize exploration risk, exploration geoche-mists are often forced to make the most out of samples either taken from outcrops near basin margins, or other available sources. Outcrop samples are commonly weathered, resulting in alteration of the organic matter. In some dark-colored but organic-lean rocks, geochemical signatures of the samples may be masked by recycled
organic matter (Farrimond et al., 1989). ``Source rocks'' taken directly from the top or bottom of the reservoirs may contain migrated hydrocarbons. Secondary altera-tion, recycled material or migrated oil unrecognized in a suspected source rock could result in a spurious oil± source rock correlation and, in turn, erroneous exploration decisions.
Peters and Moldowan (1993) recommended a number of tests to help establish whether bitumen is indigenous to the ®ne-grained rock from which it has been extracted. This note presents a case study of outcrop samples from the Shengli Hetian Block, one of the new exploration frontier areas in the Southwest Depression of Tarim Basin, NW China (Fig. 1). Useful insights were gained from the relative distributions of the potentially age speci®c C30 methylsteranes (Summons et al., 1987;
Goodwin et al., 1988; Moldowan and Talyzina, 1998). This has led to a major revision of petroleum exploration directions in this area.
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 3 9 - 5
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* Corresponding author. Fax: +1-403-292-5377.
2. Sample background and analytical methods
The Southwest Depression of Tarim Basin has a total area of about 150,000 km2, with subsidence centers near
Kashi and Yecheng-Hetian in front of the Kunlun Mountains. This study focused on Carboniferous±Permian outcrop samples collected from the Sangzhu anticlinal structure (#10, Fig. 1), which is cut through by river valleys along the south margin of the Southwest Depression, Tarim Basin. The structure is one of many E±W trending anticlines in front of the Kunlun Moun-tains, all of which were formed during the Himalayan orogeny at the end of Oligocene to Miocene. It was uplifted and exposed to surface perhaps during the Pliocene. Carboniferous±Permian strata are found in the core of the structure, with Cretaceous and Cenozoic sediments on its ¯anks. In contrast to the western seg-ment of the depression (Li et al., 1999), stratigraphic intervals containing potential petroleum source rocks in the study area occur mainly in Paleozoic strata. These include carbonates and clastics in the marine Cambrian± Ordovician and Carboniferous intervals, and in the marine to non-marine Permian strata. Limited surface geological and subsurface seismic data suggest that the study area has experienced several extended periods of uplift and erosion during the Phanerozoic. These episodes resulted in a signi®cant thinning of the Carboniferous± Permian strata and the total or partial absence of the
Middle Ordovician to Silurian and Triassic to Cretac-eous rocks.
As no wells were drilled in the eastern segment of the depression around the Hetian Sag, samples used during several previous geochemical studies were almost exclu-sively taken from outcrops. Based on conventional hopane and sterane ``®ngerprints'', a number of oil shows in the study area (from both Carboniferous± Permian and Cretaceous±Eogene intervals) were corre-lated positively to the extracts of the Carboniferous± Permian rocks (Liao Yongsheng, Zhang Linye, personal communications). Thus, these strata in the Hetian Sag were considered to be eective petroleum source rocks in this region. One exploration well was drilled on the basis of these assessments, without success.
Solid bitumens and oil stains commonly occur in many of the analyzed outcrop samples, which otherwise usually contain low amounts of organic matter (<1% TOC). It was necessary, therefore, to establish independently whe-ther the bitumens are indigenous to the Carboniferous± Permian strata and hence verify the validity of the pre-vious oil±source correlation results. For this purpose, a large number of fresh outcrop samples were collected from the same locations as in previously studies. Explo-sives were used during the collection to ensure that samples were taken at least 1.0 m below the weathered surface. For comparison, source rock cores were also taken from wells drilled in other parts of the Tarim
Basin and the adjacent Turpan Basin (Fig. 1). All avail-able data were used to ensure that the selected samples represent the best source rocks available in the Tarim Basin which also come from sedimentary facies similar to that found in the Hetian Sag area. Samples taken from the Tarim Basin include seven Cambrian rocks (black shales, limestones, dolostones and marlstones from the Tazhong and Tabei uplifts, with TOC ranging from 0.32 to 15.52%), one Lower Ordovician black shale (from the Tazhong Uplift, with 1.81% TOC), one Middle Ordovician marlstone (from the Tabei Uplift, with 2.16% TOC), 11 undierentiated Upper±Middle Ordovician samples (black shales, limestones and marl-stones from the Tazhong Uplift, with 0.32 to 12.82% TOC), and two Carboniferous black shales (from the Megaiti Slope of the Southwest Depression, with 0.57 and 1.06% TOC). Carboniferous and Permian source rocks were also taken from the adjacent Tuha Basin for comparison. These include two Carboniferous marine carbonates (with 0.60 and 1.20% TOC) and ®ve Upper Permian lacustrine shales (with 0.62 to 12.70% TOC).
Analytical conditions are de®ned elsewhere (Li et al., 1999). Brie¯y, all of the rock samples were initially screened by Rock-Eval/TOC, then extracted with dichloromethane/methanol. Saturate fractions, obtained from the rock extracts by column chromatography, were analyzed by gas chromatography (GC), gas-chro-matography/mass spectrometry (GC/MS) and gas chromatography/mass spectrometry/mass spectrometry (GC/MS/MS) using a VG 70 SQ high resolution hybrid magnetic-quadrupole instrument. In addition to com-mon hopanes and steranes, GC/MS/MS parent to daughter transitions (m/z 414!231, 414!98) were
monitored for C30 methylsteranes (including
dinoster-anes). In order to avoid co-elution problems peak heights in the mass fragmentograms were used to cal-culate certain biomarker ratios.
3. Results and discussion
Weathering is always a factor of concern for geo-chemical studies on outcrop samples. This problem was minimized in the present study of the Hetian outcrop samples, as the weathered rock surface was dynamited before samples were taken. The fact that both bulk (solvent extraction and Rock-Eval pyrolysis/TOC) and molecular parameters (GC/MS analyses) obtained in this study were comparable to those reported pre-viously, from samples collected at the same sites, sug-gests that weathering and analytical handling factors for the previous study were not signi®cant. It is worth not-ing that the characteristics of the conventional terpanes and steranes that were previously recognized using GC/ MS analyses, and used for oil-source correlation studies from the study area, was also con®rmed by the present
study. However, because the Carboniferous±Permian samples are generally lean in organic matter (typically with <1.0% TOC) and often contain solid to semi-soft fracture-®lling bitumens, the question whether the bitu-mens in these rocks are indigenous cannot be answered using the conventional biomarker ``®ngerprinting'' approach. To resolve this problem, the presence and distribution of C30methylsteranes in these samples were
investigated and compared with those in moderately to highly organic-rich rocks from other parts of the Tarim Basin and from the Tuha Basin.
Sterols bearing the dinosterane carbon skeleton occur in high concentrations in many modern dino¯agellate species and have been rarely found in other taxa (e.g. Volkman et al., 1993). Therefore, these compounds are considered to be excellent markers for dino¯agellates. Sterol precursors for 4a-methyl-24-ethylcholestanes have been found in dino¯agellates as well as other types of algae (Volkman et al., 1990), thus these compounds are less speci®c as indicators for dino¯agellates. Because 3- (and 2-) methylsteroids appear to be derived from microbial alkylation of a large variety of steroid pre-cursors related to a vast array of organisms (Summons and Capon, 1991; Dahl et al., 1992), they occur ubiqui-tously since Proterozoic times. As shown in Fig. 2, the Cambrian±Lower Ordovician source rocks from the Tarim Basin are relatively enriched in dinosteranes and 4a-methyl-24-ethylcholestanes, but these compounds are present in low abundance relative to 3b-methyl steranes in other Paleozoic source rocks examined. Such distributions do not appear to be aected by lithologies or TOC contents of the rocks. Instead, the general trends with geological age observed here parallel those reported previously using triaromatic 3- and 4-methyl-steroids (Moldowan et al., 1995). Although detailed paleontological investigation has yet to be carried out on concentrated microfossils, the relatively high abun-dance of dinosteranes can be taken as evidence for the abundant dino¯agellate ancestors in the Cambrian± Lower Ordovician strata of the Tarim Basin.
Useful information may be obtained by using the relative distributions of the C30 methylsteranes for the
dierentiation of the Cambrian±Lower Ordovician derived oils, with those having a Upper±Middle Ordo-vician or Carboniferous±Permian source. As shown in Fig. 3, the majority of the Carboniferous-Permian out-crop samples previously thought to have signi®cant source potential, and thus used for oil±source correla-tion in the Hetian area, contain C30methylsteranes that
con-tain migrated hydrocarbons derived from the Cambrian± Lower Ordovician strata. This conclusion is consistent with macroscopically visible migration characteristics of exten-sive oil shows and biodegraded solid bitumens in many of the outcrop samples.
The thick Upper±Middle Ordovician strata present in the Tazhong and Tabei depressions of the Tarim Basin are considered to be the most important petroleum source rocks in the basin (Zhang et al., 1998; Li, Xiao and Lin, unpublished results). These strata gradually thin out toward the southern part of the Southwest Depression and are even absent in the Hetian Sag area (Gu et al., 1994), where the Carboniferous±Permian
strata overlie directly on top of the Cambrian±Lower Ordovician strata. In the study area, overthrusting may have resulted also in vertical repetition of certain Car-boniferous±Permian intervals in the subsurface. Matur-ity modeling of various potential source rocks, using subsurface seismic data, indicates that the current maturity levels of the Carboniferous-Permian rocks in the Hetian Sag are too low to be eective contributors for the oil and gas shows occurring in the study area (Snowdon and Li, unpublished results). Therefore, the directions of further petroleum exploration in the east-ern segment of the Southwest Depression have to be revised, with the primary targets being placed on those structures most favorably positioned during the Hima-layan orogeny for trapping the Cambrian±Lower Ordo-vician derived hydrocarbons. In the western segment of the Yecheng-Hetian Sag (petroleum source kitchen for the Kekeya Field #7, Fig. 1) however, there is over-whelming evidence for the Carboniferous±Permian strata as being the hydrocarbon source of the Kekeya gas-condensate ®eld (Li et al., 1999). Data in Fig. 3 also suggest a clear distinction in the source characteristics between the Kekeya ®eld oils (with no detectable dinos-teranes and 4-methyl-24-ethylcholestanes) and the bitu-mens in the Hetian outcrop samples.
The results of the present investigation have far reaching implications for further oil exploration in the Paleozoic marine carbonate strata in northwestern and southern China. These indicate a clear need for a more careful, detailed check on the validity of many petro-leum resource assessment studies previously conducted
Fig. 3. Correlation of C30methylsterane ratios indicates the majority of the Carboniferous±Permian outcrop samples in the study area contain migrated hydrocarbons. See Fig. 2 for the de®nitions of the biomarker ratios. Kekeya ®eld oils and
in these areas. For example, carbonate rocks with as low as 0.1% TOC in China were previously considered as eective source rocks for commercial oil accumulations because of the vast volumes of these rocks.
Acknowledgements
We thank the Geological Survey of Canada, CNPC Tarim and Tuha Petroleum Exploration Bureaus, SINO-PEC Shengli Petroleum Bureau, Petroleum University (Beijing), Jianghan Petroleum University, Research Institute of Petroleum Exploration and Development (China), and CNPC 95 Project Oce for providing research funding and geological samples; Sneh Achal for excellent laboratory assistance; Professor Steve Larter, Drs. Mark Obermajer and Archie Douglas for com-menting on an earlier version of this manuscript. This is a Geological Survey of Canada Contribution No. 1999262.
Associate EditorÐA.G. Douglas
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