Petroleum Migration Characteristics in the Northeastern Part of the Baiyun Depression,Pearl River Mouth Basin,South China Sea

This paper investigates the origin and migration characteristics of petroleum in the northeastern part of the Baiyun Depression,Pearl River Mouth Basin(PRMB).The discovered petroleum in the study area is mainly located in the Lower Zhujiang Member(N_(1)z^(2))and mainly originated from the Enping Formation source rocks in the eastern sag.Active faults(vertical migration)and N_(1)z^(2)sandstones(lateral migration)acted as the petroleum migration systems.The fault activities in the Dongsha event controlled the episodic petroleum migration.Fractures in the fault zones provided effective conduits,and overpressure was the driving force.The vertical migration could not cross the fault zones laterally.The petroleum injection areas in the carrier beds were the contact zones of petroleum-migration faults and carrier beds.The lateral migration was steady-state migration,and buoyancy was the driving force.The migration pathways in the carrier beds were controlled by the structural morphology.Secondary petroleum migration in the study area could be divided into two parts:vertical migration along the fractures in the fault zones and lateral migration through preferential petroleum migration pathways(PPMPs)in the carrier beds.The petroleum migration behaviors,including migrating direction,driving force,and migration pattern,in the faults and sandstone carrier beds were quite different.This study provides a typical example for comprehending secondary migration processes and has great importance for determining future exploration targets in the deep-water area of the PRMB.

Preferential petroleum migration pathways and prediction of petroleum occurrence in sedimentary basins:A review

The aim of this paper is to review the major points of contention concerning secondary petroleum migration, to discuss the nature and primary controls of the positions of petroleum migration pathways in sedimentary basins, and to illustrate the importance of preferential petroleum migration pathways for the formation of large oil/gas fields away from generative kitchens. There is competition between the driving force （buoyancy） and the restraining force （capillary pressure controlled largely by permeability contrast） in controlling the positions of petroleum migration pathways in heterogeneous carrier beds. At a large scale, there is a critical angle of dip of the carrier beds which determines the relative importance of structural morphology or permeability contrasts in controlling the positions of petroleum migration pathways in heterogeneous carrier beds. Maximum-driving-force-controlled migration pathways occur in carrier beds with an angle of dip greater than the critical dip and the positions of petroleum migration pathways are controlled mainly by the morphology of the sealing surfaces. Minimum-restraining-force-determined migration pathways occur in carrier beds with an angle of dip smaller than the critical angle where permeability contrasts would exert a more important influence on the positions of petroleum migration pathways. Preferential petroleum migration pathways （PPMP）, defined as very restricted portions of carrier-beds that focus or concentrate petroleum from a large area of the generative kitchen, determine the distribution of oil and gas in sedimentary basins. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migration in hydrostatic conditions, but also for the formation of large oil or gas fields. Regions may miss out on petroleum migration because of its three-dimensional behavior, and two-dimensional migration modeling may be misleading in predicting petroleum occurrences in certain circumstances.

Petroleum Migration Direction of the Silurian Paleo-pools in the Tarim Basin, Northwest China

The results obtained in this paper indicate that carbazole-type compounds have high thermal stability and also show stability in oxidation and bio-degradation. This kind of compounds still has a high concentration and complete distribution in the analyzed dry asphalt samples, showing that they are particularly useful in studying petroleum migration of paleo-pool. During the basin＇s first-stage of oil-gas pool formation in the Silurian in Tazhong and Tabei areas of Tarim Basin （at the end of Silurian period） and the second-stage in the Awati area （in Permian）, the petroleum experienced a long-distance migration. During the formation of the Silurian paleo-pools in Tazhong Uplift at the end of Silurian, the petroleum mainly came from the lower and middle Cambrian source rocks in the Manjiaer sag. The petroleum migrated towards the southwest-south entering the Silurian reservoir beds in Tazhong first. Then, it further migrated within Silurian from northwest to southeast along the highs of the Structural Belts to the region of the Silurian pinchout boundary in Tazhong. In Tabei Uplift, during the first-stage of pool formation, the petroleum was also from the lower and middle Cambrian source rocks in the Manjiaer sag. It migrated northwest entering the Silurian reservoir beds in the Tabei Uplift firstly, and then the migration continued in the same direction within the Silurian reservoirs and finally the petroleum was trapped in higher positions. During the second-stage pool formation in the Silurian beds in the areas around Awati sag, the petroleum mainly came from the lower-middle Cambrian source rocks in the Awati sag. The petroleum migrated from the generation center to Silurian reservoirs in all directions around the sag through major paths, and the petroleum was finally trapped in higher locations.

Discussion on Petroleum Migration in Bamianhe Oilfield of Dongying Depression, East China

Petroleum migration process of Bamianhe oilfield of Dongying depression, EastChina is evaluated on the basis of our detailed work on geochemistry of the crude oil and thesource. Molecular tracers, especially nitrogen compounds, are employed in assessing style of oilmigration and accumulation. Additionally, absolute quantitative results of heteroaromatics includingdibenzofuran, dibenzothio-phene and fluorene with similar basic frames to pyrrolic nitrogeniccompounds are also used to evaluate the quantity of petroleum migration. According to apparentmigration effects of the pyrrolic nitrogen compounds, it is indicated that most of Bamianhe oil wasderived from potential source kitchen―Niu-zhuang sag, migrated and accumulated at Bamianhe faultbelt through selective paths according to a main filling point displayed. Generally, upwardmigration trend from the main point along the south slope is noted. There are observable migrationeffects from Guangli subsag to the north Bamianhe and Yangjiao-gou oilfields showing Guangli subsagwas a secondary kitchen accounting for the oils in the north portion. Significant vertical migrationeffects shown in several cross sections demonstrate that faults well developed in the area play animportant role in oil migration and accumulation. Unformality, sandstone reservoirs distributedlocally also suggest important fairways in connecting sources with traps. The tectonic and acombination of tectonic and lithological mechanism are identified as the critical mechanisms inentrapping hydrocarbons. Migration trend suggested by nitrogenic compounds agrees well with analysisof lateral compositional variety and thermal maturity gradient. This study also confirms well withour previous studies. It is the location of source rocks, tectonic and stratigraphic characteristicsthat have a comprehensive control on the model of hydrocarbon migration and accumulation inBamianhe oilfield. Results also show that the oils are mostly derived from the deep parts of theNiuzhuang and Guangli sags within normal oil window.

Complex Petroleum Migration and Accumulation in Central Region of Southern Junggar Basin,Northwest China

The central region of the southern Junggar basin （Northwest China） is a key exploration target in this petroliferous basin. As there are four sets of potential source rocks （e.g., Permian, Jurassic, Cretaceous and Paleogene sequences）, petroleum migration and accumulation are likely complex. This study represents an attempt to understand this complexity in order to provide fundamental information for future regional petroleum exploration and geological studies. Based on petroleum geology and geochemistry, it is implied that there are mainly three types of hydrocarbons, including Cretaceous- and Paleogene-sourced oils （with the former being dominant） and Jurassic-sourced gas. The petroleum migration and accumulation mainly cover three stages. The first stage is the late period of the Early Pleistocene, in which the Cretaceous-sourced oils migrate and accumulate. Then, in the second stage （from the late period of the Middle Pleistocene to the early period of the Late Pleistocene）, the Cretaceous- sourced oils, together with the Paleogene-sourced oils, participate in the migration and accumulation. At last, in the end of the Late Pleistocene, large quantities of oils remigrate and accumulate, with gas （especially Jurassic- sourced gas） migrating along faults to accumulate. Thus, petroleum charge events in the area are complex, reflecting the control of complex tectonic evolution on petroleum migration and accumulation.

Preferential Petroleum Migration Pathways in Eastern Pearl River Mouth Basin,Offshore South China Sea

Secondary petroleum migration in the eastern Pearl River Mouth basin was modeled using the three-dimensional PATHWAYSTM model, which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces. The modeling results have accurately predicted the petroleum occurrences. Most commercial petroleum accumulations are along the predicted preferential petroleum migration pathways （PPMP）, and most large fields （petroleum reserves greater than 1×10^8t） have more than one preferential petroleum migration pathways to convey petroleum to the traps. The lateral migration distance for oil in the LHII-1 field, the largest ollfield so far discovered in the Pearl River Mouth basin, was more than 80 km. The case study suggests that in lacustrine fault basins, petroleum can migrate over a long distance to form large oilfields without driving force from groundwater flow. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migration in hydrostatic conditions, but also for the formation of large oil or gas fields. The strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds with relatively large dipping angles are determined primarily by the morphology of the sealing surfaces at regional scales.

Petroleum Origins and Accumulation Patterns in the Weixinan Sag in the Beibu Gulf Basin,Using Subsag B as an Example

Crude oil has been discovered in the Paleogene and Neogene units of the Weixinan Sag in the Beibu Gulf Basin.To determine the source and accumulation mode of this crude oil,12 crude oil samples and 27 source rock samples were collected and an extensive organic geochemical analysis was conducted on them.Based on the geological conditions and the analytical results,the types,origins and accumulation patterns of crude oil in the study area were elucidated.Except for a shallowly-buried and biodegraded crude oil deposit in Neogene rocks,the crude oil samples in the study area were normal.All of the crude oils were derived from lacustrine source rocks.According to biomarker compositions,the crude oils could be divided into two families,A and B,distinctions that were reinforced by differences in carbon isotope composition and spatial distribution.Oil-source correlation analysis based on biomarkers revealed that Family A oils were derived from the mature oil shale at the bottom of the second member of the Liushagang Formation,while the Family B oils formed in the mature shale of the Liushagang Formation.The Family A oils,generated by oil shale,mainly migrated laterally along sand bodies and were then redistributed in adjacent reservoirs above and below the oil shale layer,as well as in shallow layers at high structural positions,occupying a wide distribution range.The Family B oils were generated by other shale units before migrating vertically along faults to form reservoirs nearby,resulting in a narrow distribution range.

Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin

There are three formation stages of Silurian hydrocarbon pools in the Tarim Basin. The widely distributed asphaltic sandstones in the Tazhong （central Tarim） and Tabei （northern Tarim） areas are the results of destruction of hydrocarbon pools formed in the first-stage, and the asphaltic sandstones around the Awati Sag were formed in the second-stage. The hydrocarbon migration characteristics reflected by the residual dry asphalts could represent the migration characteristics of hydrocarbons in the Silurian paleo-pools, while the present movable oil in the Silurian reservoirs is related to the iater-stage （the third-stage） hydrocarbon accumulation.

Buried Hydrothermal Systems:The Potential Role of Supercritical Water,“ScriW”,in Various Geological Processes and Occurrences in the Sub-Surface

It is well known that seawater that migrates deep into the Earth’s crust will pass into its supercritical domain at temperatures above 407°C and pressures above 298 bars. In the oceanic crust, these pressures are attained at depths of 3 km below sea surface, and sufficiently high temperatures are found near intruding magmas, which have temperatures in the range of 800°C to 1200°C. The physico-chemical behaviour of seawater changes dramatically when passing into the supercritical domain. A supercritical water vapour (ScriW) is formed with a density of 0.3 g/cc and a strongly reduced dipolar character. This change in polarity is causing the ScriW to lose its solubility of the common sea salts (chlorides and sulphates) and a spontaneous precipitation of sea salts takes place in the pore system. However, this is only one of many cases where the very special properties of ScriW affect its surroundings. The objective of this paper is to increase awareness of the many geological processes that are initiated and governed by ScriW. This includes interactions between ScriW and its geological surroundings to initiate and drive processes that are of major importance to the dynamics and livelihood of our planet. ScriW is the driver of volcanism associated with subduction zones, as ScriW deriving from the subduction slab is interacting with the mantle rocks and reducing their melting point. ScriW is also initiating serpentinization processes where olivines in the mantle rocks (e.g. peridotite) are transformed to serpentine minerals upon the uptake of OH-groups from hydrolysed water. The simultaneous oxidation of Fe2+ dissolved from iron-bearing pyroxenes and olivines leads to the formation of magnetite and hydrogen, and consequently, to a very reducing environment. ScriW may also be the potential starter and driver of the poorly understood mud and asphalt volcanism;both submarine and terrestrial. Furthermore, the lack of polarity of the water molecules in ScriW gives the ScriW vapour the potential to dissolve organic matter and petroleum. The same applies to supercritical brines confined in subduction slabs. If these supercritical water vapours migrate upwards to reach the critical point, the supercritical vapour is condensed into steam and dissolved petroleum is partitioned from the water phase to become a separate fluid phase. This opens up the possibility of transporting petroleum long distances when mixed with ScriW. Therefore, we may, popularly, say that ScriW drives a gigantic underground refinery system and also a salt factory. It is suggested that the result of these processes is that ScriW is rejuvenating the world’s ocean waters, as all of the ocean water circulates into the porous oceanic crust and out again in cycles of less than a million years. In summary, we suggest that ScriW participates in and is partly responsible for: 1) Ocean water rejuvenation and formation;2) Fundamental geological processes, such as volcanism, earthquakes, and meta-morphism (including serpentinization);3) Solid salt production, accumulation, transportation, and (salt) dome formation;4) The initiation and driving of mud, serpentine, and asphalt volcanoes;5) Dissolution of organic matter and petroleum, including transportation and phase separation (fractionation), when passing into the subcritical domain of (liquid) water.