Zhuhai Formation Research Articles

Tracing of deeply-buried source rock: A case study of the WC9-2 petroleum pool in the Pearl River Mouth Basin, South China Sea

The identification of a deeply-buried petroleum-source rock, owing to the difficulty in sample collection, has become a difficult task for establishing its relationship with discovered petroleum pools and evaluating its exploration potential in a petroleum-bearing basin. This paper proposes an approach to trace a deeply-buried source rock. The essential points include: determination of the petroleum-charging time of a reservoir, reconstruction of the petroleum generation history of its possible source rocks, establishment of the spatial connection between the source rocks and the reservoir over its geological history, identification of its effective source rock and the petroleum system from source to trap, and evaluation of petroleum potential from the deeply-buried source rock. A case study of the W9-2 petroleum pool in the Wenchang A sag of the Pearl River Mouth Basin, South China Sea was conducted using this approach. The W9-2 reservoir produces condensate oil and gas, sourced from deeply-buried source rocks. The reservoir consists of a few sets of sandstone in the Zhuhai Formation, and the possible source rocks include an early Oligocene Enping Formation mudstone and a late Eocene Wenchang Formation mudstone, with a current burial depth from 5000 to 9000 m. The fluid inclusion data from the reservoir rock indicate the oil and the gas charged the reservoir about 18–3.5 Ma and after 4.5 Ma, respectively. The kinetic modeling results show that the main stages of oil generation of the Wenchang mudstone and the Enping mudstone occurred during 28–20 Ma and 20–12 Ma, respectively, and that the δ 13C 1 value of the gas generated from the Enping mudstone has a better match with that of the reservoir gas than the gas from the Wenchang mudstone. Results from a 2D basin modeling further indicate that the petroleum from the Enping mudstone migrated upward along the well-developed syn-sedimentary faults in the central area of the sag into the reservoir, but that the petroleum from the Wenchang mudstone migrated laterally first toward the marginal faults of the sag and then migrated upward along the faults into shallow strata. The present results suggest that the trap structure in the central area of the sag is a favorable place for the accumulation of the Enping mudstone-derived petroleum, and that the Wenchang mudstone-derived petroleum would have a contribution to the structures along the deep faults as well as in the uplifted area around the sag.

Petroleum migration and mixing in the Pearl River Mouth Basin, South China Sea

Two oil groups have been investigated in the Pearl River Mouth Basin using their geochemical characteristics. In combination with source data, the two oil groups may be extrapolated into two end-member oils: petroleum populations A and B. The oil population A with abundant 4-methylsteranes, derived from the deeper Wenchang source rocks, migrated and accumulated earlier. The oil population B with absent 4-methylsteranes was expelled from the Enping source rocks and is associated with a relatively later phase of migration and accumulation. The two distinctive oil populations migrated updip through the marine blanket-like sandstone carriers within the Zhuhai Formation to accumulate in a series of traps along the main migration pathways at different times. Most of the accumulations are mixtures of the two end-member oils. The variations of 4-methylsterane concentrations in the accumulations can be related to the contributions from the two sources: the oils, which migrated furthest, contain greater contributions from the Wenchang source rocks, whereas those with shorter migration paths have greater contributions from the Enping sources. The later migrated oils closer to the depression areas are highly mature and the former oils in the Liuhua oil fields are of relatively low maturity, which may indicate the main migration direction along the Hui–Liu Structure Ridge (HLSR). Oils with abnormally high maturity in the middle of the HLSR may suggest oil-filling points, from which branch conduits connected the source kitchens to the main migration pathway. Oils with abnormally low maturities may reveal minor contributions from some small sags in the Dongsha Massif in a latest phase.

Petroleum Systems of Zhu III Depression in Pearl River Mouth Basin, South China Sea: ABSTRACT

Zhu III depression is located in the west part of Pearl River Mouth Basin, and covers an area of 11,000 sq km. Until now more than twenty wells have been drilled in the depression and its surrounding area, and all oil-gas fields and oil-gas discoveries are concentrated inside the depression. Integrated study indicates that there are two petroleum systems in Zhu III depression. One is Wenchang - Zhuhai, Zhujiang oil system which is mainly distributed in Wenchang B sag in the southwest part of the depression. Its source rock, the Wenchang formation is mainly composed of dark mudstone of lacustrine facies, with thicknesses up to more than 1000 m. Its reservoir includes tidal sandstone of transitional facies of Zhuhai formation and neritic sandstone of the lower part of Zhujiang formation. Through bounding faults and margin coarse sediment zone, oil generated from the Wenchang formation migrated into overlying sandstone of Zhuhai formation, which was overlaid by mudstone beds of bay facies of Zhuhai formation or neritic facies of Zhujiang formation, and formed oil accumulations. The other system is Enping - Zhuhai gas system, distributed in Wenchang A sag in the northeast part of the depression, whose source rock in the Enpingmore » formation deposited in the contracting stage of the lake, dominated by swamp coal measure in lithology and terrestrial plant clastics in kerogen components. The gas generated from Enping formation directly migrated into overlying tidal sandstone of Zhuhai formation and formed gas accumulations. Therefore, exploration in Wenchang A sag in the northeast part of the depression is for gas accumulations, and oil accumulations in Wenchang B sag in the southwest part of the depression, while oil-gas mixed accumulations are likely to be found in the transitional area of two systems.« less

The depositional characteristics and oil potential of paleo Pearl river delta systems in the Pearl river mouth basin, South China Sea

Delta systems in the Zhuhai, Zhujiang and Hanjiang formations are interpreted as having formed during periods of sea-level rise from the late Oligocene to mid-Miocene (30-10.5 My B.P.). Deltas in the Zhuhai Formation were developed in a shallow water environment on a sandy, wave- or fluvial-dominated coasts. Thick and blanket-like sandstones are common but mudstones are infrequent. Oil-bearing zones have been found only in the upper part of the systems, overlain by mudstone of the Zhujiang Formation. Deltas in the Zhujiang Formation are interpreted as having formed in a deeper water environment on sandy, wave- or fluvial-dominated coasts. Carbonate platforms started growing on the Dongsha massif as the sea-level slowly rose. The moderate sandstone/mudstone ratio in the delta systems and a zone of secondary porosity in the carbonate rocks provide an excellent reservoir rock and seal rock for the basin. Deltas in the Lower Hanjiang Formation are similar to those of the Zhujiang Formation, but the upper part of the Formation is interpreted as having formed in a shelf environment; the thick shelf mudstone, interbedded with offshore bar sandstones, would be one of the better regional seal rocks in the study area. No carbonate rock developed on the Dongsha massif due to the rapid rise in sea-level. All delta systems from the Zhuhai to the Zhujiang Formation are stacked in an onlap pattern indicating a more and more expanding marine transgression, which was caused by eustatic sea-level rise. This marine transgression is almost unique and contrasts to the progradational (offlap) pattern of Cenozoic delta systems in the Gulf of Mexico. It resulted in thick sandstone deposits, immature or poorly mature source rock and no ductile mudstones, with no growth faults or rollover structures. Hydrocarbons generated from Eocene-Oligocene lacustrine source rocks are trapped by drape structures related to local basement highs or by carbonate rock with secondary porosity. Most of the remaining undiscovered reserves in the basin will be found in non-structural, subtle traps, especially stratigraphic traps.

Geology and geochemistry of source rocks of the Eastern Pearl River Mouth Basin, South China Sea

The Pearl River Mouth Basin (PRMB), with mostly Cenozoic sediments, is one of the largest offshore oil-bearing basins in the South China Sea. It covers an area of 175,000 km 2 with the maximum sedimentary thickness of 12 km. Large structural belts trending NE-SW were developed in the basin. From north to south they are the Northern Fault Terrace-Uplift, Northern Depression, Central Uplift and Southern Depression Belts, respectively. The evolution of the PRMB had been in two stages, i.e. first faulting (from Paleocene to Early Oligocene) and then subsiding (starting from Late Oligocene), and the so-called Double Structural Layers were thus developed. Therefore, the sedimentary rocks of the PRMB are characterized by terrestrial followed by marine facies. Three terrestrial (accendingly known as Shenhu, Wenchang and Enping Formations, respectively) and six marine formations (accending named Zhuhai, Zhujiang, Yuehai, Wanshan and Quaternary Formations, respectively) were deposited. Geochemical statistical study of possible source rocks in the PRMB showed that both the terrestrial Enping and Wenchang Formations were richer in organic matter while both the marine Zhujiang and Zhuhai Formations are relatively poor. The type of organic matter is dominantly composed of Types II to I for the Wenchang Formation and mainly Type III for the other three overlying formations. Furthermore, in thermal maturity, the Zhujiang Formation is either immature or marginally mature, Zhuhai, marginally mature-mature, Enping, mature-highly mature and Wenchang, highly-overmature. Hence, the principal source rocks in the PRMB are the Wenchang and Enping Formations, and the Zhuhai and Zhujiang Formations could also be oil sources. Geochemical investigations indicated that the source rocks in the PRMB are characterized by lower saturates to aromatics ratio, commonly found 18a(H)-oleanane and resin-derived “W” and “T” compounds in some source rocks, enriched high molecular weight n-paraffin hydrocarbons and C29 steranes except for those of Wenchang and partly of the Enping Formations and predominant C30 hopane for Wenchang Formation in contrast to the others, which contained either predominant C31 homohopane or C29 norhopane or C27 trisnorhopane. However, the more abundant C30 4-methyl steranes discovered in crude oils of the PRMB were only identified from a thin setting of Wenchang Formation occurring in Huilu Lowhigh. Therefore, it was deduced that there must have been a certain setting of the Wenchang Formation which contains enriched C30 4-methylsteranes which are widely distributed in the depositional centers of the separated Depressions of the PRMB. They are possible oil sources of the reservoirs found in the PRMB.

Petroleum Geology of the Zhu-1 Depression, Pearl River Mouth Basin, People's Republic of China: ABSTRACT

The Pearl River Mouth basin, located in the South China Sea between Hainan Island and Taiwan has been the focus of an intense exploration effort during the l980s. In 1979 the international oil industry, acquired over 60,000 km of seismic, gravity, and magnetic data covering an area of approximately 240,000 km{sup 2}. Three major subbasins, Zhu-1, Zhu-2, and Zhu-3 were defined. Chevron in partnership with Texaco and AGIP (ACT group), concentrated their effort on the Zhu-1 depression which was interpreted to contain as much as 7,800 m of sedimentary section. This subbasin, bounded by the Wansha and Donsha massifs to the north and south, is the most inboard of the three depressions, thereby possibly prolonging anoxic lacustrine conditions prior to the Neogene marine incursion. Additionally, the Zhu- 1 depression should have directly received Miocene sediment potentially supplying the subbasin with high-quality reservoirs. Within the Zhu-1 depression, the ACT group focused in on Block 16/08, which covered the deepest part of the Zhu-1 depression. The block was awarded to the consortium in January 1983. Structuring within the block ranges from Paleogene tensional block faulting created during the early formation of the overall Pearl River Mouth basin to draping over basement highsmore » and carbonate buildups during the Neogene. The Pearl River Mouth basin exhibits classic rift basin geometry with early nonmarine continental fluvial/lacustrine deposition (Zhuhai Formation) during the Oligocene and capped by a lower Miocene marine incursion (Zhu Jiang Formation). Integrated interpretations, exploration drilling, and constant refinement of the geological model led to the discovery of two oil fields, Huizhou/21-1 and Huizhou/26-1, both of which are currently under development and will represent the first commercial oil production from the entire Pearl River Mouth basin.« less

Depositional Features and Source and Reservoir Rocks of Tertiary Age in Northern Part of South China Sea: ABSTRACT

The northern part of the South China Sea covers an area of about 400,000 km/sup 2/. Tertiary deposits more than 10,000 m in thickness are widely distributed there. The area has sedimentary rocks more than 1000 m thick covers more than 300,000 km/sup 2/. Five sedimentary basins have been established in this area: Beibu Bay, Yinggehai, Southeastern Qiong, Pearl River Mouth, and Southwestern Taiwann basins. The primary source and reservoir rocks for oil and gas prospects are of Tertiary age. Tertiary rocks underwent three stages of development, each forming a specific sedimentation system: (1) a system of fluviolacustrine deposits in rift depressions from the Paleocene to early Oligocene; (2) a system of semiclosed-sea deposits from the late Oligocene to early Miocene; and (3) a system of deltaic open-sea deposits from the middle Miocene to Pliocene. These three sedimentation systems resulted in three suites of source rocks, three suites of reservoir rocks, and three groups of independent oil pools, complete with source, reservoir, and cap rocks. The three suites of source rocks are as follows: (1) the Eocene Liushagang Formation in the Beibu Bay basin, which is believed to be the best source rock discovered in the area; (2) the Oligocenemore » Zhuhai Formation in the Pearl River Mouth basin; and (3) the lower Miocene series in the Pearl River Mouth basin. The Eocene formation is probably the principal source rock of regional scale in the northern part of the South China Sea. The three suites of reservoir rocks are as follows: (1) the fluviolacustrine sandstone bodies in the Liushagang Formation; (2) the fluviolacustrine sand bodies and shallow-sea sandstone bodies in the Zhuhai Formation and Lingshu Formation; (3) the deltaic, littoral, and shallow-sea sand bodies and bioherms of Neogene age, with the middle Miocene sandstone reservoirs having the best physical properties.« less