Hydrocarbon Accumulation Mechanisms Near The Top Overpressured Surface In Central Junggar Basin, Northwest China

Covering an area of about 1.36×10⁵ km², the Junggar Basin is located in the northern part of the Xinjiang Uygur Autonomous Region, NW China, which is one of the most prolific oil provinces in China. As both a lowest explorated area and also a more potential area in Junggar Basin, the study area is seated in the West Pen1 Well Depression, North Dongdaohaizi Depression and Changji Depression in the central Junggar Basin. Deep zone overpressure system is commonly developed in central Junggar Basin (deeper than 4000 meters) and many drillings met the overpressure zone. As probably hydrocarbon-originated and post-diagenesis-stage overpressure in central Junggar Basin, oil and gas were mainly accumulated around the top overpressured strata. Studying the reservoirs near the top overpressured surface in central Junggar Basin, based on description of overpressured strata distribution and analysis of overpressure origin and evolution, to research into the chemistry characteristics of coal-bearing source rocks and hydrocarbon, the hydrocarbon distribution characteristics, the main types of reservoirs and its formation processes and so on in detail, will help to better understand the processes of hydrocarbon migration, fluid-rock interaction, secondary porosity formation and hydrocarbon accumulation controlled by such overpressured system. The thesis is divided into four parts as follows.1 The geophysical response characteristics and its mechanisms, origin, distribution and evolution of overpressure in central Junggar BasinThe measured abnormally high pressures of the sandstone layers in central Junggar Basin reveal the depths generally ranging from 4470m to 6160m. The overpressured layers of sandstones are mostly in the Jurassic Formation, only a few sections are in the bottom of the Cretaceous Formation. The excess pressures (formation pressure minus hydrostatic pressure) range from approximately 11 MPa to 57MPa with the pressure coefficients (formation pressure divide by hydrostatic pressure) of 1.24 to 2.07. The measured porosity and permeability for overpressured sandstone samples are mostly lower to very lower.Marked increases of the densities of drilling mud in the fluid overpressured zones are presented, and the responses of exceptionally high sonic and low resistivity logs relative to their normal trends in the overpressured shales and sandstones can be observed. The values of vitrinite reflectance in the large overpressured zones of Jurassic Formation range from about 0.55-1.0%. The observed data suggests that the top of large overpressured zones may not be shallow at depth of 4400m, and the tops in some drilling well are to reach as deep as about 6000m. The depth distributions of large overpressured zones are controlled by the burial depths of the Jurassic Formation (mature source rocks). This study indicated that the main cause of overpressures in the Jurassic Formation of full compaction is hydrocarbon generation-related. The physical simulation experiments show that the effective stress of rock framework reducing is due to high pore fluid pressures, which can directly lead to the decrease of velocity of acoustic wave through the rocks, that is the higher interval transit times responding to overpressuring rather than to higher porosities. Under the overpressured formation temperatures, the ionization constant of high-pressure liquid water (near-critical water) may be increased, which appear to decrease formation resistivity.The depth of overpressure strata in central Junggar Basin differs in different depressions. The West Pen1 Well Depression has the depth from 4 kilometers to 7 kilometers, and the North Dongdaohaizi Depression and the east part of Changji Depression have the depth from 5 kilometers to 9 kilometers, and the center and west of Changji Depression have the depth from 6 kilometers to 10 kilometers. Most of the overpressure systems were along-bed distributed, and layer penetration could be found in the margin area and some local areas of the depression. Those strata transferred from the top Triassic to the bottom of Cretaceous.The overpressure evolution is divided into two stages. The early stage began Late-Jurassic or Early-Cretaceous generated by the transferred overpressured fluid from bottom overpressured strata, and the overpressure strength gradually declined to hydrostatic pressure in Late-Eogene. After Late-Eogene, the overpressure strength ascended as a result of amounts of hydrocarbon generation when the Jurassic coal-bearing strata maturated, and the pressure coefficient could reach 1.6 to 1.9 present day.2 The geochemistry characteristics of coal-bearing source rocks and hydrocarbon, and the analysis of hydrocarbon accumulation processes near the top overpressured surface in central Junggar BasinThe most important source rocks are the Lower-Permain and the Middle-Permain lacustrine facies mudstones, and the Jurassic coal-bearing mudstone is the secondary important source rocks in central Junggar Basin.Based on the organic geochemistry datum of the 30 crude oil and oil-bearing sandstone samples, 1 natural gas sample, and other collected data, several available index marks differing the hydrocarbon source from the Permain source rocks and the Jurassic source rocks can be summed up, such as crude oil carbon isotopic values, Pr/Ph values, tricyclic diterpane distribution content and their internal distribution types, homosterane distribution types, gammaceran index, if have carrotane and tri-aromatic sterane or not, and et al. The vitrinite reflectance calculated from many maturity indexes, such as MPI1, 4,6-/1,4-DMDBT,αααC₂₉S/(S+R), C₂₉ββ(αα+ββ), and so on, is ranging from 0.7% to 1.0%, and is in the maturity stage generating amounts of liquid hydrocarbon, in which the maturity of Permain source rocks is a little higher. The natural gas is mainly comprised of coal-bearing wet gas, and the calculated maturity is mainly between 0.9% and 1.0%.The distributions of different sourcing hydrocarbon in different tectonic units are different. In the central Junggar Basin, the Lower-Permain source rock began to generate hydrocarbon (R_o=0.5%) at about late Early-Permain, reacheed the hydrocarbon generation peak(R_o=1.0%) at about Late-Permain, and quickly came to the high maturity(R_o=1.3%) later; comparing with the Lower-Permain source rock, the time scale of hydrocarbon generation of the Middle-Permain is longer, from late Middle-Permain to the Late-Cretaceous; the Jurassic coal-bearing source rock began to generate hydrocarbon at about late Early-Jurassic, but the maturity values differs each other in different tectonic units at present day, respectively, West Pen1 Well Depression 0.7-1.0%(R_o), North Dongdaohaizi Depression and Changji Depression 0.8-1.2%(R_o). It experienced two stages of hydrocarbon accumulation, and there are two times of great hydrocarbon charge in the late stage of hydrocarbon accumulation in the central Junggar Basin. 3 The main types of reservoirs and its formation processes near the top overpressured surface in central Junggar BasinFrom J₁b to J₂t, the sedimentary depositions are comprised of four delta regime prograding sequences.Based on the evidences of the litho-electrical assemblage feature, paleontology, clay minerals, stratigraphic contact relationships in the strata beneath J/K unconformity, it can prove the existence of J₂t in Middle-Che-Mo Palaeo-Uplift in central Junggar Basin, and this fact indicates there are various stratigraphic traps in study area.The Che-Mo Palaeo-Uplift formed in Middle-Late-Jurassic and experienced six evolution stages.Fluid and rock interact frequently in the deep buried sandstones near the top overpressured surface in central Junggar Basin, and commonly distributed carbonate cements record abundant information on the episodic overpressured fluid flow in geo-history. Through characteristics analysis mainly on the carbon-oxygen isotopic in carbonate cements, combined with the reservoir petrography, fluid inclusions, and other data, in deep buried sandstones near the top overpressured surface in central Junggar Basin, several conclusions can be summed up as follows: (1) ferroan sparry carbonate cements formed in Late Diagenesis Stage are the uppermost fillings in the reservoir; (2) co-variation ofδ¹³C andδ¹⁸O of carbonate cements in sandstone are apparently lighter negative values, and are concentrated in finite areas close related with organic influence; (3) carbonate cements are mainly products of thermal fluid movement in Late Diagenesis Stage; (4)regarding the top overpressure as the boundary, the co-variation ofδ¹³C andδ¹⁸O in sandstone carbonate cements are apparently lighter negative values, showing such law as: lighterδ¹³C vs.δ¹⁸O near the coal-bearing Jurassic stratum, heavierδ¹³C vs.δ¹⁸O in the top overpressure, and lighterδ¹³C vs.δ¹⁸O upper the top overpressure, which is considered to be the results of isotopic kinetics fractionation derived from episodic overpressured fluid flow; (5) secondary porosities, dissolved by episodic overpressured acid fluid (organic acid and CO₂) from Lower-Jurassic and Middle-Jurassic coal-bearing stratums, are the most important reservoir space for hydrocarbon accumulation.4 The hydrocarbon accumulation mechanisms and the advantageous exploration target prediction near the top overpressured surface in central Junggar BasinHydrocarbon was mainly accumulated in a narrow areas, 300 metres above and 100 meters below the top overpressured strata in central Junggar Basin .In central Junggar Basin, as a hydrocarbon-originated and post-diagenesis-stage overpressured basin, source rocks are in the overpressured compartment, the top overpressured surface is both a tight-lithostatic and unpermeable seal surface and a balance surface of hydrocarbon migration. The thermal evolution of source rocks, the spatial composition of the transportation pathways, the evolution of the overpressure, the characteristics of the top overpressured seal, and the types and distributions of the advantageous traps and reservoirs, are all the controlling factors of the hydrocarbon accumulation processes and the hydrocarbon accumulated areas distribution near the top overpressured surface in central Junggar Basin. Close related with the evolution of the overpressure, the traps and the reservoirs near the top overpressured surface experienced two stages of hydrocarbon accumulation, and there are two times of great hydrocarbon charge in the late stage of hydrocarbon accumulation in the central Junggar Basin.The traps near the spots where the overpressured organic fluid released are the advantageous exploration targets near the top overpressured surface in central Junggar Basin.