Zhuhai Formation Research Articles

Stratigraphic architecture, shelf-edge delta and constraints on the development of the Late Oligocene to Early Miocene continental margin prism, the Pearl River Mouth Basin, northern South China Sea

The early-stage shelf-margin prism in the Pearl River Mouth Basin (PRMB), northern South China Sea, formed from the Late Oligocene to Early Miocene, comprises a set of shelf-edge delta and slope fan wedges of the Zhuhai Formation and records the development of the continental slope in response to the interaction of sea-level change, tectonic subsidence and sediment supply. Based on integrated analysis of seismic, well logging and core data, the stratigraphic architecture, depositional systems and controlling processes are documented. The Zhuhai Formation is a composite sequence bounded by regional unconformity, and can be further divided into five sequences (CS3-1-CS3-5). The depositional genetic units (transgression, ascending normal regression and descending forced regression) are identified within sequences based on variation in shelf edge trajectories. The shelf deltaic clinoforms are characterized by oblique-tangential reflections in seismic profiles with a thickness of 100–150 m. In contrast, the shelf-margin delta and slope clinoforms generally are 300–1200 m thick and mainly display as sigmoid or sigmoid-tangential seismic reflections. The delta deposits usually consist of thick beds of delta front deposits and prodelta slump and deformed sediments, and are associated with sandy slope fan systems. Eleven delta complexes are distinguished by tracking the distribution of the deltaic clinoforms in each sequence, and these deltaic deposits tend to extend along their strike direction which may be related to the reworking of strong waves and coastal currents. Slope fan systems consist mainly of turbidite channel fills, frontal splay and debris flow deposits, and generally develop along the slope in CS3-3-CS3-5, which may be triggered by steep shelf margin slope and fault activity. The stratigraphic pattern of the shelf margin prism and depositional evolution of the deltaic systems from the Late Oligocene to Early Miocene were controlled by the interaction of sea level changes, sediment supply and tectonic subsidence. The ascending trajectory from the CS3-1 (30 Ma) to the top of the CS3-5 indicates that the relative sea level continued to rise generally but the shelf margin prograded basinward rapidly due to the great amount of sediment supply and lower subsidence rate. Tectonic uplift of the source area and strengthened physical erosion during this period may be the reason for the large sediment supply.

Petroleum charge history in the Baiyun depression and Panyu lower uplift in the Pearl River Mouth Basin, northern South China Sea: Constraints from integration of organic geochemical and fluid inclusion data

The source, thermal maturity, and charge history of oil and gas condensate in the Baiyun depression and the Panyu lower uplift were determined using organic geochemical and fluid inclusion data. Nearly all the gas condensates and oil in the study area originated from Eocene Enping Formation source rocks, whereas the condensate fluids in the eastern area originated from the Enping and Zhuhai Formations. The condensate fluids have a relatively low thermal maturity with a calculated vitrinite reflectance equivalent that typically ranges from 0.8% to 1.0% based on the C29 sterane isomerization ratios and aromatic hydrocarbon maturity parameters. Four types of oil inclusions with various fluorescence colors (yellow–blue) and API gravities (25°–44°) were recognized in the Oligocene and lower Miocene reservoir units, which suggest multiple oil charging events. The homogenization temperatures of the fluid inclusions recorded the oil charge events accompanying large-scale hot fluid movement, which was most likely caused by the charging of high-temperature natural gas from the deeper Wenchang Formation. The trapping pressures of the oil inclusions and the reconstructed burial histories were used to determine the timing of oil charge. The main oil charge occurred from 16–7 Ma throughout the Baiyun depression and Panyu lower uplift, whereas the Panyu lower uplift experienced an additional oil charge from 5–0 Ma. The natural gas charge occurred during the main oil charging period and may continue to the present day. Multiple physical processes, such as gas injection (without gas exsolution), gas washing (with gas exsolution), and migration fractionation, have played important roles in gas condensate formation and controlling the petroleum distribution in the Baiyun depression and Panyu lower uplift.

Diagenesis and reservoir quality in tight gas bearing sandstones of a tidally influenced fan delta deposit: The Oligocene Zhuhai Formation, western Pearl River Mouth Basin, South China Sea

Abstract Deeply buried Oligocene Zhuhai sandstones are important gas exploration targets in the western Pearl River Mouth Basin, South China Sea. The tight sandstones are characterized by low porosity (less than 10%), low permeability (less than 1 mD) and strong microscopic heterogeneity due to diagenesis during their geologic history. Petrographic analysis is undertaken based on cores, thin sections, X-ray diffraction and scanning electron microscope, that is helpful to understand the facies, lithologic characteristics, diagenesis and their impact on reservoir quality. Six lithofacies were defined in the Zhuhai sandstones, namely bedded granule-rich coarse sandstone to granule conglomerate, cross-bedded medium-to coarse-grained sandstone, cross-bedded fine-to coarse-grained sandstone, structureless sandstone, interbedded sandstone and siltstone and claystone and shale. They were accumulated in the fan delta systems and tidal flat systems. The sandstones are dominated by feldspathic litharenite, litharenite, sublitharenite and quartz arenite. Major portion of the porosity is due to secondary pores and primary pores are relatively less dominant. Primary pores are more important to tight sandstones, well preserved with moderate compaction and weak cementation. Well-sorted, medium-to coarse-grained sandstones with rigid quartz and rock fragment grains commonly have high primary porosity. Early cements such as chlorite rims are helpful for primary porosity preservation. The dissolution of feldspars in the Zhuhai Formation helps to increase the total porosity, whereas it also decreases the permeability and spoils the tight reservoir quality.

Detrital zircon geochronology reveals source-to-sink relationships in the Pearl River Mouth Basin, China

A major challenge in provenance and source-to-sink analysis is deciphering intrabasinal versus extrabasinal sediment sources and processes. This issue is well-illustrated by the Paleogene Pearl River Mouth Basin (PRMB), whose provenance reflects a combination of sediments eroded from the Cathaysia Block (CB), southeastern China, and from intrabasinal structural highs. Here we use new detrital zircon UPb ages from Paleogene formations in seven boreholes, southern PRMB, to investigate such local versus regional influences on provenance. Detrital zircons from the upper Eocene-lower Oligocene Enping Formation are dominated by Jurassic-Cretaceous ages, with a 125–110 Ma age-cluster, absent in modern river data from the CB and diagnostic of intrabasinal sediment sources. In the northern part of the basin, Mesozoic-dominated zircon age-spectra give way to Paleozoic and Precambrian grains, a pattern also recognized for the late Oligocene Zhuhai Formation, suggesting a major change in provenance likely related to extrabasinal influence from the CB. The age spectra for these samples are most similar to the northeastern Pearl River, narrowing this region as the most likely source. Spatial analysis of the results suggests that during deposition of the Enping Formation, sediment transport was not uniform and that the basin's architecture strongly influenced provenance, with structural highs acting as both sediment sources and barrier to sediment transport from the CB; whereas structural lows, with clear extrabasinal influence, functioned as sediment transport corridors. Such spatial differences are lost in samples from the Zhuhai Formation and their provenance suggests that the paleo-Pearl River and tectono-climatically driven surface processes in the CB dominated deposition thereafter.

Sequence architecture and depositional evolution of the late Palaeogene in southwestern slope of Lufeng Depression, Pearl River Mouth Basin: Responses to tectonic processes and base‐level changes

The southwestern slope of Lufeng Depression, which is located in the central part of Zhu I Depression is a significant geomorphic unit due to its petroliferous discoveries and tectonic evolutions since early Palaeogene. Based on the comprehensive analysis of cores, wireline logs, seismic volumes, and other related geological and geophysical information, the sequence architecture and depositional evolution was analysed, and their responses to regional tectonic processes and base‐level changes were revealed. The results indicated that two second‐order sequences and six third‐order sequences defined by different types of sequence boundaries are recognized. The second‐order and third‐order sequences were bounded by regional erosional unconformities on the depression margin or slope and unconformities in the central depression with the assist of stacking patterns of wireline logs and depositional characteristics of cores. The second‐order sequence generally consists of two regional transgressive–regressive cycles which can be further subdivided into several local sub‐cycles equivalent to corresponding third‐order sequences which can be identified from wireline logs and cores. The palaeogeomorphology resulted from compound and complex tectonic events in the early Palaeogene plays an important and main role in controlling sequence architecture. In contrast, the base‐level changes largely controls the sequence architecture in the late Palaeogene. The depositional systems within the sequence stratigraphic framework are mainly composed of braided delta deposits and shore deposits, which respectively includes braided channel, subaqueous distributary channel, lacustrine shale, beach bar, maximum flooding surface, shoreface, and foreshore. The base‐level changes, which apparently originated from tectonic uplift and subsidence and sediment supply, controlled the regional transgressions and regressions in sedimentary facies distributions. The development of subsequence (what we call it sand units or sections) from 2,980 to 2,880 in Enping Formation in the Early Palaeogene is obviously influenced by a decrease in tectonic subsidence (rifting stage) and an increase in accommodation space (base‐level rising), while the case is opposite in subsequence 2,850 of Enping Formation. However, the subsequence in Zhuhai Formation in the late Palaeogene from 2,760 to 2,600 was formed in stable tectonic settings (transition stage), and thereby, it was mainly dominated by base‐level changes which were caused by sediment supply and accommodation space changes.

Detrital zircon provenance record of the Oligocene Zhuhai Formation in the Pearl River Mouth Basin, northern South China Sea

Abstract The Oligocene Zhuhai Formation has been proven to be one of the main reservoirs in the Pearl River Mouth Basin in the northern South China Sea but its sedimentary provenance remains uncertain and controversial. Here we conducted a systematic study by U-Pb analysis of detrital zircon of the Zhuhai Formation in order to identify the source terrane, investigate the sedimentary source, and establish the framework for the provenance variation during the Oligocene. The study suggests that the detrital zircon in the different structural units reveal marked spatial differences in their age compositions that correlate with the changing sources. The western and eastern parts of the basin have a significantly different provenance manifested by the age compositions of the detrital sediments. The western basin generally has a relatively complex age spectrum and is characterized by a multi-peak distribution, suggesting that the main source was derived from the southern margin of the South China Block. In contrast, the eastern basin displays a dominant population ranging from 131 to 142 Ma, which implies that the sediments were mainly eroded from a nearby source. Our data demonstrate that the Cathaysia Block hinterland and the intrabasinal sources provided dominant provenance contributions to the Pearl River Mouth Basin during the Oligocene. Additionally, Hainan Island and Central Vietnam (eastern Indochina Block) do not appear to have been important sources, whereas Luzon Island and northern Palawan probably provided negligible contributions to the basin during the same time period.

Sequence architecture and depositional evolution in Lufeng Depression, Pearl River Mouth Basin, South China Sea Part B: The shore deposits of Zhuhai Formation

The Zhuhai Formation in Lufeng Depression is mainly deposited during a depression stage. In this paper, we employ cores, wireline logs, and seismic reflections to reveal sequence architecture and depositional evolution in target intervals. The results indicated that one second‐order sequence and two third‐order sequences were identified and sequence stratigraphic framework was established. The second‐order sequence was developed in response to base‐level falling, while third‐order sequences were mainly formed during base‐level rising. In addition, each third‐order sequence did not change too much in thickness. Then the shore deposits (shoreface and foreshore) were delineated, and the depositional characteristics were described within the framework with the help of related data. The shoreface and foreshore deposits within the sequence stratigraphic framework varied significantly in stacking patterns of wireline logs and cores. Based on comprehensive analysis, it can be concluded that base‐level changes played an important role in controlling sequence architecture. And lacustrine fluctuations derived from accommodation space changes exerted an essential effect on depositional evolution. This study provides significant insights into revealing the sequence architecture and depositional evolution and discussing hydrocarbon potential in the transition stage in the Zhuhai Formation.

Geochemical characteristics and analysis of crude-oil source in the deep-water area of the Baiyun Sag, South China Sea

In the Baiyun Sag, the oil relationship with possible source rocks is complicated, and little research on oil sources in the area has been performed. In this paper, geochemical studies of carbon isotopes, biomarkers, paleoenvironmental deposition, and crude oil maturity, carried out with the use of oil samples from the Baiyun Sag, demonstrate that the organic matter in crude oil has an oxidizing depositional environment and primarily lacustrine facies conditions, with a significantly higher plant input. The oil samples used are mature to highly mature. Crude oil in the northern area of the Baiyun Sag is more mature than samples in the eastern part of the sag. Despite limited source rock data, we were able to conclude that the crude oil samples were derived mainly from the Enping Formation; other potential source rocks of the Wenchang and Zhuhai Formations may also have contributed to the reservoirs in different parts of the sag. Furthermore, the source of crude oil in each structure in the Baiyun Sag was analyzed; for example, crude oil in the LH19-5 structure was generated from the organic matter of the Enping Formation, and crude oil in the LW3-1 structure was derived from the mixed organic matter of the Enping, Wenchang, and Zhuhai Formations. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Petrography and geochemistry of Oligocene to Lower Miocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea: Provenance, source area weathering, and tectonic setting

There is a controversy regarding the source area and weathering intensity of Oligocene to Lower Miocene sediments in the Baiyun Sag of the Pearl River Mouth Basin. Detrital modes, geochemistry, heavy mineral data, and detrital zircon U–Pb ages were analysed to understand the provenance, source area weathering, and tectonic setting of Oligocene to Lower Miocene sandstones in the Baiyun Sag. Petrographic data reveal that the sandstones were derived from a recycled orogen. With increasing transport distance, sediment maturity increased correspondingly. The Oligocene to Lower Miocene sandstones in the Baiyun Sag have low compositional maturity and have experienced low to moderate weathering. The sediments of the Zhuhai Formation have experienced relatively strong weathering intensity compared with those of the Enping and lower Zhujiang formations. Due to low degree of illitization and differences in the geochemical compositions of the sediments, the continental island arc data from different boreholes in the Baiyun Sag are more irregularly distributed in the A‐CN‐K plot. The modal composition (e.g., quartz, feldspar, and lithic fragments) and geochemical indices (Al2O3/TiO2 ratio; Al2O3‐TiO2 diagram; discriminant F1‐F2 diagram; Eu/Eu*, La/Sc, La/Co, Th/Sc, and Th/Co ratios; Hf‐La/Th and La/Sc‐Co/Th diagrams; and rare earth element patterns) indicate that the sediments in the Baiyun Sag are mainly derived from felsic source rocks. Although the uplift around the Baiyun Sag includes multistage magmatism and basic rocks, it has little influence on the sediment in the depression, and no sediments were transported by the Han River to the Baiyun Sag. Geochemical data and detrital zircon U–Pb ages suggest that the Pearl River was formed in the Early Oligocene and became the main source for the sediments in this area. The river carried felsic material from the South China Coast forming a large delta on the northern slope of the Baiyun Sag. The ratios of Eu/Eu*, La/Sc, La/Co, Th/Sc and Th/Co, and heavy mineral data indicate that the sediments transported by the Pearl River to the Baiyun Sag were changed from the arc‐dominated coastal zone to the interior of the South China Block during the Late Oligocene to Early Miocene and even reached the south‐eastern margin of the Tibetan Plateau (West Yunnan Plateau). The La‐Th‐Sc and Th‐Sc‐Zr/10 diagrams suggest that the provenance of these sandstones is evolved continental island arc or an active continental margin setting.

Diagenesis-porosity evolution and “sweet spot” distribution of low permeability reservoirs: A case study from Oligocene Zhuhai Formation in Wenchang A sag, Pear River Mouth Basin, northern South China Sea

Abstract The characteristics of low permeability reservoirs and distribution of sweet spots in the Oligocene Zhuhai Formation of Wenchang A sag, Pearl River Basin were investigated by core observation and thin section analysis. The study results show that there develop the fine, medium and coarse sandstone reservoirs of tidal flat – fan delta facies, which are of mostly low permeability and locally medium permeability. There are two kinds of pore evolution patterns: oil charging first and densification later, the reservoirs featuring this pattern are mainly in the third member of Zhuhai Formation between the south fault zone and the sixth fault zone, and the pattern of densification first and gas charging later is widespread across the study area. Strong compaction and local calcium cementation are the key factors causing low permeability of the reservoirs in the Zhuhai Formation. Thick and coarse grain sand sedimentary body is the precondition to form “sweet spot” reservoirs. Weak compaction and cementation, dissolution, early hydrocarbon filling and authigenic chlorite coating are the main factors controlling formation of “sweet spot” reservoir. It is predicted that there develop between the south fault and sixth fault zones the Class I “sweet spot” in medium compaction zone, Class II “sweet spot” in nearly strong compaction zone, Class III “sweet spot” reservoir in the nearly strong to strong compaction zone with oil charging at early stage, and Class IV “sweet spot” reservoir in the strong compaction and authigenic chlorite coating protection zone in the sixth fault zone.

Using Detrital Zircon Geochronology to Constrain Paleogene Provenance and Its Relationship to Rifting in the Zhu 1 Depression, Pearl River Mouth Basin, South China Sea

AbstractPaleogene syn‐rift successions in the South China Sea are poorly understood and systematic provenance analysis, which could provide clues to their history, is lacking. Here we report 409 new concordant U‐Pb ages from detrital zircons separated from the Paleogene Wenchang, Enping, and Zhuhai formations in the Zhu 1 depression, Pearl River Mouth Basin. The new data, combined with the published age data from the region, document changes in the provenance of syn‐rift successions. Detrital zircons from the Eocene Wenchang Formation are unimodal, with Jurassic‐Cretaceous (180–80 Ma) ages making up >80% of grains. The ages are consistent with the geochronology of intrabasinal highs, dominated by igneous rocks emplaced during the Yanshanian orogeny, and suggest local provenance. By contrast, detrital zircons from the upper Eocene to lower Oligocene Enping Formation form three well‐recognized age‐clusters, with peaks at 150, 254, and 438 Ma that match documented tectonomagmatism in South China Block (SCB). Combined with increasing numbers of Precambrian zircons, the data suggest increasing influence of regional provenance of the SCB. Similar age peaks are also recognized from the limited number of zircons analyzed from the upper Oligocene Zhuhai Formation and comparability with modern shelf and river sediment indicates the unit was mainly sourced from the SCB and likely transported by a paleo‐Pearl River. We infer that the change in provenance, from local uplifts within the Zhu 1 to the SCB, is related to distinct phases of PRMB rift development; however, later changes are best explained by SCB drainage evolution.

Characteristics and Hydrocarbon Generation of the Marine Source Rock of the Upper Oligocene Zhuhai Formation in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

The Baiyun Sag, located in the northern South China Sea, is rich in oil and gas. The marine mudstone in the Upper Oligocene Zhuhai Formation of the Upper Oligocene has not previously been considered as the main source rock and was thus selected for a thermocompression simulation experiment to verify its hydrocarbon generation potential. The experimental results were used to calculate the hydrocarbon generation kinetic parameters, and the hydrocarbon generation process of the marine mudstone was simulated. The mudstones of the Upper Oligocene Zhuhai Formation in the deepwater area of the Baiyun Sag are effective source rocks. The dark shallow marine mudstones of the Upper Oligocene Zhuhai Formation from well C generate hydrocarbons at a rate of 179.79 kg/ton of total organic carbon. As a set of active source rocks, the Upper Oligocene Zhuhai Formation has reached the low mature to mature stage and mainly generates oil in the marginal areas of the Baiyun Sag, whereas they have reached a highly overmature st...

Diagenetic differences caused by the charging of natural gases with various compositions - a case study on the lower Zhuhai Formation clastic reservoirs in the WC-A sag, the Pearl River Mouth Basin

Only a limited number of comparative studies have explored the diagenetic differences of reservoirs caused by the charging of natural gases with different compositions during the migration and accumulation of oil and gas. In this study, we quantitatively assessed the diagenetic variations of the lower Zhuhai Formation reservoirs in the WC-A sag, the Pearl River Mouth Basin, as a result of natural gas charging with different compositions. The employed methods included electron microscopy, scanning electron microscopy, cathode luminescence, X-ray diffraction, chemical composition analysis of formation water, stable isotope analysis, and fluid inclusion determination. The results indicated that: (1) in the lower Zhuhai Formation reservoir that are near the large fractures, the late-charging of CO2-rich thermal fluid promoted the dissolution of minerals, and changed the porosity, permeability, pH value of fluid, and the contents of ions such as K+, Na+, Mg2+ of the reservoirs. These changes, in turn, indirectly affected the content, form, and distribution of clay minerals in the reservoirs. In addition, the secondary enlargement of quartz was enhanced. (2) The charging of CO2-rich thermal fluid caused strong dissolution and produced high contents of cements. The dissolution mainly occurred in a half open system with strong fluid activity. Consequently, the significantly decreasing of permeability was hindered and middle porosity–middle permeability reservoirs were generated. The reservoirs that are far from the large fractures, however, were mainly early charged by the hydrocarbon-rich fluid. The dissolution was weak and the generated cements were limited. In addition, the dissolution products could not be discharged in such closed diagenetic systems. As a result, the permeability of the reservoir was declined sharply and resulted in low porosity–low permeability reservoirs.

The effect of river-delta system on the formation of the source rocks in the Baiyun Sag, Pearl River Mouth Basin

Source rock formation influenced by river-delta system, especially in continental margin basins, is still poorly understood. This article aimed to reveal the effect of river-delta system on the formation of the source rock by taking the Baiyun Sag of the Pearl River Mouth Basin for example. Paleo-Pearl River began to develop since the Enping Formation, providing abundant organic matter beneficial for the formation of the source rocks in the Baiyun Sag. The main controlling factor of source rock formation in the Baiyun Sag is terrestrial organic matter supply rather than the paleoproductivity or redox conditions. Low Al/Ti and P/Ti ratios suggest low marine productivity, which may be associated with a large number of terrigenous detritus input, occupying about 43.04%–94.91%. There is a positive correlation between the oleanane/C30hopane ratio and the TOC value, showing that terrigenous organic matter controls the source rock formation. The size of the delta below Pearl River estuary determines the extent of terrestrial organic matter supply. Source rocks with high organic matter abundance mainly formed in delta environment, and those in neritic environment in Enping and Zhuhai Formations also have high TOC values as a result of adequate terrestrial organic matter supply.

Reservoir characteristics in the LW3-1 structure in the deepwater area of the Baiyun sag, South China Sea

Geochemistry including carbon isotopes, biomarkers, and maturity indexes indicates that the oil and gas in the Liwan 3-1 (LW3-1) structure are derived from source beds in the Enping, Wenchang, and Zhuhai formations (fms.). The major contribution is from the Enping fm. Two major sandstone reservoirs in the LW3-1 structure are present in the early Miocene deepwater turbidite fan of the Zhujiang fm. and the late Oligocene delta front sediments of the Zhuhai fm., where most traps are structural traps and drape anticlines. Faults act as the main migration pathways while permeable sandstone layers are favorable lateral passage systems. Structures formed by diapirism are conducive to oil and gas migration. A combination of faults, the location of the diapir belt, and the structural ridge control the oil and gas migration and accumulation. Sand units 1–3 in the LW3-1 structure share a common temperature-pressure system and gas-oil contact; they are considered to be the same oil- and gas-bearing strata set. Oil and gas injection intensity is higher in sands 1 and 2 than in sands 3 and 4, in which sand 4 is separated from the upper three reservoirs and belongs to a different oil- and gas-bearing strata set. Oil and gas injected in sands 1–3 migrate vertically along faults, and then migrate laterally to accumulate in highly permeable sands. In sand 4, oil and gas migrate along faults; they accumulate in the trap and form a reservoir with better perseveration of oil and gas compared with the others.

Influences of Tibetan Plateau uplift on provenance evolution of the paleo-Pearl River

A comparative analysis of the geochemical characteristics of sediments from the Oligocene Zhuhai Formation (32–23.8 Ma), the Miocene Zhujiang Formation (23.8–16.5 Ma), and the Hanjiang Formation (16.5–10.5 Ma) and a comprehensive analysis of the geochemical characteristics of rocks surrounding the paleo-Pearl River drainage contribute to understanding the influences of the Tibetan Plateau uplift on provenance evolution of the paleo-Pearl River. The results show that the geochemical characteristics of sediments from the Oligocene Zhuhai Formation are very different from the geochemical characteristics of sediments from the Miocene Zhujiang and Hanjiang Formations. The ∑ rare earth elements (REE) of mudstone is relatively high in the Zhuhai Formation, 204.07–293.88 ppm (average 240.46 ppm), and low in the Zhujiang and Hanjiang Formations, 181.32–236.73 ppm (average 203.83 ppm) and 166.84–236.65 ppm (average 199.04 ppm), respectively. The chemical index of alteration (CIA) for these samples has a similar trend to the ∑ REE: the CIA of the Zhuhai Formation is relatively high and the CIA of the Zhujiang and Hanjiang Formations is relatively low. The uplift of the Tibetan Plateau is crucial to the westward expansion of the paleo-Pearl River drainage.

The Tidal Sandstone Characteristic of the Zhuhai Formation, Huizhou Oil Field, Pearl River Mouth Basin, South China Sea

abstract The Huizhou oil field is a prolific producer of hydrocarbons from the Oligocene Zhuhai in the Pearl River Mouth Basin. Previous studies characterized the Zhuhai Formation as braided delta, which is not supported by this study. We did a detailed core description about 170 m (558 ft) and recognized seven lithofacies: (1) tidal bedding siltstone; (2) bidirectional cross-stratification sandstone; (3) mud-draped sandstone; (4) rhythmic graded bedding sandstone; (5) tangential contacts and mud-draped toesets sandstone; (6) reactive surface sandstone; and (7) parallel and cross-parallel stratification sandstone. These are evidence of tidal sedimentation. Tidal sandstone develops at the seaside, which is opposite of shoreface sandstone.

High-resolution sequence stratigraphic division and distribution of tidal deposits in the Zhuhai Formation, Huizhou Sag, Pearl River Mouth Basin, South China Sea

AbstractThe Zhuhai Formation of the Huizhou Oilfield has been interpreted as being deposited in a tide-dominated estuarine setting. This paper uses stratigraphic principles, cores and logging data to analyse the facies changes of tidal sedimentation in response to the base level cycle. We conclude that the mud flat progradational allocycle is the result of a long-term base level fall, while the fining-upwards autocycle is in response to a short-term base level fall. Region-wide mud layers of supratidal zones and scour surfaces of subtidal zones are stratigraphic surfaces of tidal sedimentation. By integrating high-resolution stratigraphic correlations with seismic attributes extracted from the stratal slicing of 3-D data volumes, we use an attribute map of RMS (root mean square) amplitude to image the distribution and evolution of tidal deposits in the study area. Intertidal zones and the upper parts of the tidal cycle are favourable areas for prospecting and oil reserves and thus provide a basis for petroleum exploration and development.

Chlorite cement and its effect on the reservoir quality of sandstones from the Panyu low-uplift, Pearl River Mouth Basin

Based on porosity and permeability measurements, mercury porosimetry measurements, thin section analyses, SEM observations, X-ray diffraction (XRD) analysis and granulometric analyses, diagenetic features of reservoir sandstones taken from the Zhuhai formation in the Panyu low-uplift of the Pear River Mouth Basin were examined. This study shows that chlorite cements are one of the most important diagenetic features of reservoir sandstones. The precipitation of chlorite was controlled by multiple factors and its development occurred early in eo-diagenesis and continued till Stage A of middle diagenesis. The precipitation of chlorite at the early stage was mainly affected by the sedimentary environment and provenance. Abundant Fe- and Mg-rich materials were supplied during the deposition of distributary channel sediments in the deltaic front setting and mainly in alkaline conditions. With the burial depth increasing, smectite and kaolinite tended to be transformed into chlorite. Smectite cements were completely transformed into chlorite in sandstones of the studied area. Volcanic lithics rich in Fe and Mg materials were dissolved and released Fe2+ and Mg2+ into the pore water. These cations precipitated as chlorite cements in middle diagenesis in an alkaline diagenetic environment. Chlorite coatings acted as porosity and permeability, thus helping preserve cements in the chlorite cemented sandstones. The reservoir quality of chlorite cemented sandstones is much better than sandstones without chlorite cements. Chlorite cements play an important role in the reservoir evolution that was mainly characterized by preserving intergranular porosity and forming better pore-throat structures of sandstones.

Dating petroleum emplacement by illite 40Ar/39Ar laser stepwise heating

The timing of petroleum emplacement is crucial in understanding petroleum-forming processes and predicting new prospecting regions. This study investigates the possibility of determining the age of petroleum emplacement by an illite 40Ar/39Ar laser stepwise heating technique. A specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields, resulting in credible 40Ar/39Ar data. The sandstone samples are from the Zhuhai and Zhujiang formations in the Huizhou sag in the Pearl River Mouth Basin, South China Sea. Illite 40Ar/39Ar laser stepwise heating consistently yields gradually rising age spectra at the low-temperature steps until reaching age plateaus at high-temperature steps. The first-step apparent ages of the six samples from the Zhujiang formation give a weighted mean age of 11.4 0.5 Ma (1), in agreement with an age of 12.1 1.1 Ma in the first three steps of the sample from the Zhuhai Formation. These dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the Huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation. The plateau ages ranging from 58 to 276 Ma in high laser-output steps are interpreted as being caused by variable contributions from detrital K feldspar in the sandstones. The older total ages based on the illite 40Ar/39Ar stepwise heating results are equal to the K-Ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital K feldspar. The 40Ar/39Ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional K-Ar methods.