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Abstract
The Kongdian Formation in Wangjiagang structural zone in Dongying depression is complex in structural characteristics, and its tectonic evolution process is still poorly understood. To solve these problems, various data including seismic, logging and core were integrated to analyze the structural characteristics of Kongdian Formation, summarize its regularity of formation and evolution, and reveal the control effect on hydrocarbon accumulation with rose diagram of faults, balanced cross section and fault-sealing analysis methods. The following results were obtained. Most faults were formed in the DongyingKongdian period. Owing to the differences of formation mechanisms, faults with different trends are different in timing and configurations. On plane view, the faults show configurations such as comb-like, parallel or echelon. While on sectional view, they show configurations such as step, "Y" shape or roof-like. Folded belts which were controlled by Tan-Lu fault zone and basement strike slip fault system have undergone three evolutionary stages including formation stage, intensification stage and finalization stage. During this period, the folded belts were cut by earlier or growth faults, resulting in moniliform fault-nose distribution. The structural zone controlled hydrocarbon pooling. It is the most favorable destination of hydrocarbon migration and has better physical properties of reservoir in the axial region than those in two wings. Fault sealing and carrier beds are the key factors controlling hydrocarbon migration and accumulation. The length of fault, the width of folded zone and reservoir physical property determine the plane distribution area of oil/gas, while the fault configuration and amplitude of fold zone jointly control the spatial distribution of oil/gas.
... Generally, they are comprised of a series of band-distributed en-echelon, arc-shaped or composite fault blocks. If they possess favourable petroleum geological conditions, such as oil sources, reservoirs, channelling and sealing, they could form string-like, en-echelon or band-like oil reservoirs, which are also indicators of concealed fault zones, such as the Wang Jiagang and Ba Mianhe structural zones in the Dongying sag, China (Song et al., 2013). They are both small-scale concealed fault zones with widths less than 1 km and lengths 15 km, respectively, and induced by Paleogene dextral strike slips of NE basal faults ( Figure 1). ...
Concealed fault zones are a type of weak deformation fault zones that form in sedimentary covers and have expressive forms. This paper introduces an effective way to identify such structures according to the distributions of reservoirs, sedimentary facies, sub-sags (or sags) and salients (or ridges), and classified concealed fault zones into five types according to their scales, i.e., trap-scale, sub-sag-scale, sag-scale, depression-scale and basin-scale and characterised their structural features. Concealed fault zones go through five consecutive evolutionary stages, from the initiation of induced fractures (R shears), to the development of localised en echelon faults (R fault), to the appearance of discontinuous main faults (Y faults), to interconnected Y faults that accommodate the strike slips of basal faults, and finally to a through-going revealed strike-slip fault. Case studies show that oil-gas reservoirs often distribute as en echelon, string-or band-like features in concealed fault zones, so they might be promising oil-gas exploration targets in the future.
The Wangjiagang (WJG) fault zone is regarded as the northward propagation of the Tan-Lu fault zone (TLFZ) in the Bohai Bay Basin (BBB). We used three-dimensional seismic and borehole data, as well as structure and isopach maps, to describe the structural properties and tectonic development of the WJG fault zone. The properties and thickness of the mechanical layer are the principal factors of differences in the deformation of the strike-slip fault zone. Extensional tectonics and strike-slip faulting occurred in WJG. Not only does strike-slip faulting regulate sand body distribution, but it also plays a critical role in oil migration. WJG provides a record of the tectonic evolution of the BBB and how it responded to plate interactions around the Pacific-Eurasian-Indian plates and/or the strike-slip faulting of the TLFZ. The BBB extended in the early Paleocene as a result of relative relaxation between the Pacific and Eurasian plates and the upwelling of the asthenosphere mantle. Before and after the late Eocene (ca. 42 Ma), the tectonic stress regimes at a nearly N-S-oriented extensional to dextral strike-slip faulting along TLFZ coincided with the strike-slip faulting of the pre-existing fault in WJG. The Pacific Plate's change in direction and rate of subduction, as well as the far-field effect of a hard collision between the Indian and Eurasian plates, caused the TLFZ's two walls to move in opposite directions. The resulting force couple caused a strong simple shear in the TLFZ and adjacent areas, including the WJG fault zone.
Ductile, clay-rich sand grains control porosity and permeability in the fluvio-deltaic Oligocene and lower Miocene sandstones of the South China Sea. Ductile grains account for between approximately 5 and 50% of the original sand grain population. There is a pronounced loss of porosity with increasing burial depth in the basin, At depths of less than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss with depth increases with increasing ductile grain content. At depths greater than 3000 m, the steep porosity loss with depth is due to combined ductile grain compaction and quartz cementation. The amount of quartz cement increases with increasing burial depth; however, cleaner sandstones tend to have greater amounts of quartz cement at any given depth below 3000 m. This leads to convergence of porosity evolution for the clean and ductile-rich sandstones below 3000 m, There is a rapid loss of permeability with decreasing porosity because cam paction of ductile grains smears them between rigid quartzose grains leading to blocked pore throats. A consequence of this process is that the lowest permeabilities are found in sandstones with the highest ductile grain contents. Quartz cement does not have a clear and discernible control on permeability, The pronounced loss of porosity with increasing depth and permeability with decreasing porosity leads to low permeability at relatively shallow burial depths. Reservoir quality is thus controlled by the nature of the primary sand and depth of burial. The sediment supply system led to systematic changes in ductile grain content across the basin with ductile content increasing into the more distal part of the sediment system. The consequence is that depth of economic basement (in terms of porosity or permeability) can be predicted as a function of ductile grain content and burial depth far prospects across the basin.
There are strong heterogeneous characteristics of reservoir property between the core and limbs of the Kela-2 anticline, although they are in the same structure. The reservoir heterogeneity mode of “small-scale and east-west” cannot be explained by the distribution mode of compressional stress “large-scale and south-north” proposed by previous scholars. Statistical result of petrographic composition and diagenetic characteristics shows that the differentiation of reservoir compaction between the core and limbs of the Kela-2 anticline is clear. The differentiation is controlled by the difference of tensile stress suffered by strata above the neutral plane during folding deformation. The tensile stress suffered by strata in the anticlinal core is stronger than that in the anticlinal limbs. In the Kela-2 anticline, the difference of tensile stress between the anticlinal core (Kela-201) and limbs (Kela-203 & Kela-204) is obvious. The tensile stress offsets the compaction effect caused by vertical and lateral (south-north) stress, and it is constructive to reservoir quality. It is the major factor controlling the differentiation of reservoir compaction.
This paper proposes a new dynamic model to explain the evolution of the Tanlu fault zone, that is, the multiforce-in-multiphase acting (on the Tanlu fault zone) model. In the Early and Middle Jurassic, two kinds of forces, the lasting convergent force(NNE-SSW orientation) after Late Triassic collision of Yangtze and North China blocks and the force (NW sense) from paleo-Pacific plate, acted on the paleo-Asian continental margin (Andes-type) to produce a compressional and sinistral strike-slip displacement of the Tanlu fault zone. In Late Jurassic, the northern paleo-Asian continental margin converted to the West Pacifictype, and the Pacific Plate's subduction induced the upper mantle's upward movement on the middle and northern Tanlu fault zone, which resulted in an extensional and dextral displacement of the Tanlu fault zone. The similar action did not take place on the southern section of the Tanlu fault zone until Late Cretaceous. Since Late Cenozoic, the Pacific Plate's subduction has withdrawn and its induced upper mantle's upward movement also has withdrawn to the present Marginal Sea area. The NWW-sense compression deriving from sub-convection of the upper mantle under the above Marginal Sea area makes the Tanlu fault zone slip compressionally and dextrally.
The understanding of fault seal generation is crucial to the assessment of the migration and trapping of hydrocarbons. The effectiveness of the seal depends upon the porosity and permeability characteristics of the fault zone, which is controlled by the microfabrics present. The range of processes which effect the microfabric evolution and the development of seals in fault zones are reviewed. Three broad classes of seal may be recognised: (1) collapse seals, where the permeability/porosity of the fault zone is reduced by pore volume decrease achieved by grain reorganisation produced by fracturing, grain deformation and dissolution and by grain boundary sliding; (2) cement seals, where the reduced flow across a fault is achieved by the precipitation of phases in and adjacent to the fault; (3) juxtaposition seals, generated by the juxtaposition of lithologies with different sealing capacities. -from Author