Es3 Source Rocks Research Articles

Quantitative prediction of mixed-source crude oils and its significance for understanding oil accumulation in subtle pools in the Dongying Depression, Bohai Bay Basin

Conventional geochemical approaches were utilized in the quantitative prediction of the proportions of mixed-source crude oils derived from the Es3 and Es4 members of the Paleogene Shahejie Formation. The mixed-source oils are accumulated in the middle interval of the Es3 member (Es3M) in the Niuzhuang Sag, which is one of the sags where subtle traps are primarily of lenticular basin-floor turbidite sands within mudstones mostly developed in the Dongying Depression, Bohai Bay Basin. The result showed that about 18–92% of the mixed-source oils were derived from the Es4 source rocks with an average of 55–60%. Reservoirs associated with deep faults appear to have much more Es4 genetic affinity oils. A high proportion of the Es4-derived oils discovered in the Es3M subtle lithological traps in the Niuzhuang Sag have long migration distances. This suggests that surrounding source rocks might not necessarily control the hydrocarbon supply for subtle traps. Subtle migration pathway may play an important role in the vertical oil migration. The traditional concept of hydrocarbons accumulation in these lenticular turbidite sandstone traps within a short migration distance from the surrounding source rocks is not supported by this study. The present result is also consistent with our previous findings that immature oils in the Bamianhe Oilfield in the south slope of the Niuzhuang Sag were actually mixed-source oils mostly sourced from the Es4 in the depocenter of the Niuzhuang Sag, and the petroleum potential of the Es4 member in the Dongying Depression should therefore be re-evaluated.

Origin of Oils in “Subtle pools” in the Dongying Depression, Bohai Bay Basin, China

Abstract: Subtle traps or oil pools have become an important exploration play in the Dongying Depression, Bohai Bay Basin, east China. Despite recent successes in exploration, the formation mechanisms of subtle traps are still not well understood. The majority of subtle oil pools in the Dongying Depression are developed in the middle interval of the Es3 Member of the Paleogene Shahejie Formation with the subtle traps being primarily of lenticular basin‐floor turbidite sands encompassed in mudstones. Oil in the subtle traps was previously thought to have migrated directly from the surrounding source rocks of the same formation (Es3). Detailed geochemical investigation of 41 oils and 41 rock samples from the depression now indicates that the oils from the subtle traps cannot be correlated well with the surrounding Es3 source rocks, which are characterized by high Pr/Ph (>1), low Gammacerane/C30hopane, representing a freshwater lacustrine setting. In contrast the oils features low Pr/Ph (<1) and relatively high Gammacerane content, showing a genetic affinity with the underlying Es4 source rocks, which also have the same qualities, indicating a brackish lacustrine setting. Oils in the Es3 subtle traps are probably derived from mixed sources with the contribution from the upper Es4 source rocks predominating. Therefore unconventional oil migration and accumulation mechanisms need to be invoked to explain the pooling of oils from the Es4 source rocks, which probably came through a thick low interval of the Es3 source rocks with no apparent structural or stratigraphic pathways. We suggest that the subtle oil migration pathway probably plays an important role here. This finding may have significant implications for future exploration and the remaining resource evaluation in the Dongying Depression.

Organic compositions of lacustrine source rocks in Jiyang super-depression and its implication to petroleum geology

The compositions of organic matter in four immature source rocks from Tertiary strata of Jiyang super-depression, the most typical continental rift subsidence basin in East China, have been studied by different extracting methods with CHCl3, MAC and CS2/NMP, respectively. The results suggest that there are great differences among the chemical compositions of organic matter in the source rocks derived from different depositional environments. About 79% of all the organic matter exists by non-covalent bond in the Es4 source rocks which were deposited under the saline lacustrine, indicating that its organic matter is not the real kerogen, but mainly composed of soluble organic matter which is easy to generate hydrocarbon at lower temperature. This is why the immature oils were derived from Es4 source rocks in Dongying depression. In contrast, around 60% of organic matter exists by covalent bond in Es3 source rocks which were deposited under the deep brackish-fresh lacustrine, showing that Es3 source rocks are mainly composed of kerogen producing mature hydrocarbon at higher temperature. The thermal simulation experiments, upon the remaining solid source rocks which were sequentially extracted by the three solvents, have been carried out. The chloroform extracts from the simulation product have been compared with the other three solvent extracts gained at room temperature. It is obvious that remarkable odd/even predominance (OEP) is mainly the characteristic of soluble organic matter; phytane mostly exists in the soluble organic matter by means of non-covalent bonds and characteristics of soluble organic matter are similar to these in immature oils produced in Jiyang super-depression.

THE GENERATION OF IMMATURE OILS IN THE LACUSTRINE JIYANG MEGA‐DEPRESSION, BOHAI BAY BASIN, CHINA

The lacustrine Jiyang mega‐depression (Bohai Bay Basin, eastern China) produces important volumes of immature oils. These oils were generated at shallow burial depths by source rocks which have not entered the conventional oil window. Oil‐source correlation studies have determined that the oils were mainly generated by source rock intervals in the Es4 and Es1 Members of the Paleogene Shahejie Formation, which were deposited in saline‐ to brackish‐water and brackish‐water lacustrine conditions, respectively. Minor quantities of oil came from Es3 Member source rocks deposited in freshwater conditions.Samples of the Es4, Es3 and Es1 source rocks underwent artificial maturation at low temperatures (100–400°C). For the Es4 sample, the hydrocarbon generation characteristics of soluble organic matter and kerogen were investigated. The results of the experiments show that soluble organic matter made a greater contribution than kerogen to the immature oil (76% versus 24%). A comparison of the results of the thermal simulation experiments with the naturally‐occurring maturation profile shows that various factors influenced the generation of the immature oils. Firstly, the Es4 and Es1 Members include intervals enriched in algal material. Secondly, most of the immature oil is derived from soluble organic matter. Finally, the saline depositional environment was favorable for the preservation of organic matter.

Analysis of wax hydrocarbons in petroleum source rocks from the Damintun depression, eastern China, using high temperature gas chromatography

A detailed organic geochemical study; utilising petrography, biomarker hydrocarbon analysis and high temperature GC analysis of extractable wax hydrocarbon constituents was performed on four marginally oil window-mature source rocks from the Shahejie Formation (Eocene), Damintun depression in eastern China. The main maceral components in the source rocks were vitrinite, liptinite and exinite, with vitrinite being more abundant (>50 vol.%) in organic-lean samples whose TOC contents were between 1 and 2 wt.%. Large differences in pristane/phytane ratios suggested that the organic-rich samples were deposited in a less oxic depositional environment than that for the organic-lean rocks. The distribution of extractable wax hydrocarbons, determined by high temperature GC, showed a marked difference between these two sample types. The organic-rich samples contained high molecular weight hydrocarbons (HMWHCs) dominated by macrocrystalline n-alkanes ( n-C 23– n-C 37, typically maximising at n-C 29), while the organic-lean samples contained lower amounts of extractable wax hydrocarbons but were relatively rich in microcrystalline components (> n-C 35). In all source rocks (Es3 and Es4), a noticeable odd-over-even predominance (OEP) of n-alkane chain lengths (up to n-C 65) was evident, consistent with a direct biological origin for the long n-alkyl chains. They were most probably formed during diagenesis from decarboxylation of predominantly even-carbon-numbered aliphatic acids originating from higher plant or lacustrine algal sources and/or were directly biosynthesised in hydrocarbon form. At least two other homologous series of branched/cyclic HMWHCs were observed, one of which was confirmed as a series of branched alkanes (probably methyl-branched). The carbon number distribution patterns of HMWHCs may be primarily controlled by thermal maturity and biogenic source input as well as being influenced by diagenetic reactions governed by depositional environmental conditions, as shown previously [Carlson, R.M.K., Teerman, S.C., Moldowan, J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatography of high wax oils. In: Indonesian Petroleum Association, 22nd Annual Convention Proceedings. Jakarta, Indonesian, pp. 483–507. Carlson, R.M.K., Jacobsen, S.R., Moldowan, J.M., Chan E.I., 1994. Potential application of high temperature gas chromatography to Middle Eastern petroleum exploration and production. In: Al-Husseini, M.I. (Ed.), Geo'94, Vol 1., Selected Middle East Papers from The Middle East Petroleum Geoscience Conference, 1994; Gulf PetroLink. Manama, Bahrain, pp. 258–267]. Our study indicates for the first time that Es3 source rocks as well as Es4 facies contain HMWHCs. The distributions of extractable wax hydrocarbons suggest that both Es4 and Es3 members may potentially serve as important parent source rocks for generating waxy petroleum in this region.

97/01742 Subcritical CO2 retorting vs supercritical CO2 extraction of oil shale

The Fulin basin is located within the giant Shengli petroleum province in East China. The source rock in the basin is mainly the lower part of the Third Member (Es3) of the Eocene portion of the Shahejie Formation. It is composed of organic-rich dark mudstone and oil shale of lacustrine origin and contains good quality Type I–II kerogen. These strata reach the oil-generative stage at Ro = 0.4-0.8%. The First Member (Es1) of the Oligocene portion of the Shahejie Formation has excellent oil potential in the nearby Gunan basin but is immature in the Fulin basin and is not an effective source rock.The oil and rock samples from both basins can be grouped into three major families based on biomarkers. The first family shows low PrPh (< 1), a high gammacerane index (> 0.3) and notable β-carotane, typical of carbonate-evaporite environments. This family exists in Es1 rocks and the related oils and is early mature [ββ(ββ + αα) = 0.2-0.3, and S(R + S) = 0.10 – 0.20]. The second family shows high PrPh (> 1), a low gammacerane index (< 0.2) and abundant 4-methyl steranes, indicative of freshwater lacustrine conditions like those in Es3. This family shows higher maturation [ββ(ββ + αα) = 0.30-0.50 and S(R + S) = 0.35-0.55] and notable diasteranes. The third family is transitional, has source and maturity ratios between the above families, and occurs in the Es3 unit samples from the basin interior.Based on biomarker similarities between oils and rock extracts, all oils in the Fulin basin originated from Es3 source rocks. Slight maturation differences indicate that these oils have been derived from different source areas. No oil from Es1 is found in the Fulin basin, suggesting Es1 source rocks are too immature to generate commercial oil. Although the nearby Gunan basin has much oil generated from Es1 source rocks, no migrated oil from Es1 in this basin reached the Kenli faulted zone because of intervening faults which disrupt migration.

Oil-source correlation in the Fulin basin, Shengli petroleum province, East China

The Fulin basin is located within the giant Shengli petroleum province in East China. The source rock in the basin is mainly the lower part of the Third Member (Es3) of the Eocene portion of the Shahejie Formation. It is composed of organic-rich dark mudstone and oil shale of lacustrine origin and contains good quality Type I–II kerogen. These strata reach the oil-generative stage at R o = 0.4-0.8%. The First Member (Es1) of the Oligocene portion of the Shahejie Formation has excellent oil potential in the nearby Gunan basin but is immature in the Fulin basin and is not an effective source rock. The oil and rock samples from both basins can be grouped into three major families based on biomarkers. The first family shows low Pr Ph (< 1), a high gammacerane index (> 0.3) and notable β-carotane, typical of carbonate-evaporite environments. This family exists in Es1 rocks and the related oils and is early mature [ ββ (ββ + αα) = 0.2-0.3 , and S (R + S) = 0.10 – 0.20 ]. The second family shows high Pr Ph (> 1), a low gammacerane index (< 0.2) and abundant 4-methyl steranes, indicative of freshwater lacustrine conditions like those in Es3. This family shows higher maturation [ ββ (ββ + αα) = 0.30-0.50 and S (R + S) = 0.35-0.55 ] and notable diasteranes. The third family is transitional, has source and maturity ratios between the above families, and occurs in the Es3 unit samples from the basin interior. Based on biomarker similarities between oils and rock extracts, all oils in the Fulin basin originated from Es3 source rocks. Slight maturation differences indicate that these oils have been derived from different source areas. No oil from Es1 is found in the Fulin basin, suggesting Es1 source rocks are too immature to generate commercial oil. Although the nearby Gunan basin has much oil generated from Es1 source rocks, no migrated oil from Es1 in this basin reached the Kenli faulted zone because of intervening faults which disrupt migration.