Hydrocarbon Accumulation Pattern Research Articles

North‐south Differentiation of the Hydrocarbon Accumulation Pattern of Carbonate Reservoirs in the Yingmaili Low Uplift, Tarim Basin, Northwest China

Abstract: By analyzing the characteristics of development, structural evolution and reservoir beds of the residual carbonate strata, this study shows that the residual carbonate strata in the Yingmaili low uplift are favorable oil and gas accumulation series in the Tabei (northern Tarim uplift) uplift. There are different patterns of hydrocarbon accumulation on the northern and southern slopes of the Yingmaili low uplift. The north‐south differentiation of oil reservoirs were caused by different lithologies of the residual carbonate strata and the key constraints on the development of the reservoir beds. The Mesozoic terrestrial organic matter in the Kuqa depression and the Palaeozoic marine organic matter in the Manjiaer sag of the Northern depression are the major hydrocarbon source rocks for the northern slope and southern slope respectively. The hydrocarbon accumulation on the northern and southern slopes is controlled by differences in maturity and thermal evolution history of these two kinds of organic matter. On the southern slope, the oil accumulation formed in the early stage was destroyed completely, and the period from the late Hercynian to the Himalayian is the most important time for hydrocarbon accumulation. However, the time of hydrocarbon accumulation on the northern slope began 5 Ma B.P. Carbonate inner buried anticlines reservoirs are present on the southern slope, while weathered crust and paleo‐buried hill karst carbonate reservoirs are present on the northern slope. The northern and southern slopes had different controlling factors of hydrocarbon accumulation respectively. Fracture growth in the reservoir beds is the most important controlling factor on the southern slope; while hydrocarbon accumulation on the northern slope is controlled by weathered crust and cap rock.

Key geological controls on migration and accumulation for hydrocarbons derived from mature source rocks in Qaidam Basin

The mature oil/gas source rocks exist mainly in Jurassic strata in the northwest and in Tertiary strata in the west of Qaidam Basin. Oil and gas generated from the mature hydrocarbon source rocks in the Jurassic and Tertiary mainly accumulate in Tertiary formations in the Qaidam Basin. The main sedimentary facies in Tertiary formations include alluvial plain, lakeshore, shallow lake, semi-deep lake, alluvial fans, fan delta and sublacustrine fan. However, there were no fluvial facies. Most of the alluvial plain facies and lakeshore facies are very narrow and they become wide only where basin-bounding faults are present. Most parts of the basin are filled with shallow lake sediments such as pelitic siltstone and silty sandstone. The deposition systems have determined the hydrocarbon migration and accumulation patterns in Tertiary formations in different parts of the basin. In the inner basin where continuous permeable formations are absent, faults and fractures are the main pathways. Fractured rocks are the main reservoir types. Along the basin margins where continuous permeable formations occur in abundance, faults and fractures are also the main pathways. Therefore, the faults and fractures are very important for the hydrocarbon migration and accumulation. Four migration and accumulation models have been developed based on this study: (1) Migration and accumulation model through thrust faults. This model is mainly for migration and accumulation at the basin margin. The mudrock associated with the major fault is about 10 m thick so that there exist the hanging wall and footwall migration and accumulation zones. Where the major fault extends into source rocks areas, hydrocarbon migrated to and accumulated in the reservoir in the hanging wall. Otherwise, the faults just seal the reservoir in the footwall. (2) Migration and accumulation model through combined syn-depositional reverse faults and strike-slip faults. This model is also mainly for migration and accumulation at basin margins in which the strike-slip faults control different kinds of sedimentary fans and connections with hydrocarbon source areas; they are very suitable for hydrocarbon accumulation. (3) Migration and accumulation model through recoil faults. This model is mainly for migration and accumulation in fractured reservoirs in the inner basin region. (4) Migration and accumulation model through superficial thrust fault and lower strike-slip fault complex. This is mainly for migration and accumulation in the north of Qaidam Basin where the reservoirs are at the footwall of the superficial thrust faults.

Migration and accumulation of hydrocarbons in the Swiss Molasse Basin: implications of a 2D basin modeling study

This 2D modelling study attempted to quantify the processes of hydrocarbon (HC) generation, expulsion and migration along a regional section (NW–SE) in the North Alpine Foreland Basin of Western Switzerland. Modelled excess pressures increase towards the Alpine front, and are mainly related to the lithology distribution; excess pressure compartments are centred around shaly or evaporitic intervals of low permeability. By late Jurassic times, a first major phase of HC generation is initiated in the deepest part of the Permo-Carboniferous grabens in the external part (i.e. NW) of the Molasse basin. A second more important generation phase starts in Oligocene–Miocene times in the internal parts close to the Alpine front. Migration of HC seems to be controlled predominantly by the layer-cake geometry of the Mesozoic passive margin sequence. In contrast to many studies where vertical buoyancy driven migration dominates, the main driving mechanism for the migration and accumulation of HC is the excess pressure evolution. Large overpressure zones in the frontal part of the orogen (i.e. Subalpine Molasse) can drive deep fluids far updip into the foreland. The build-up of overpressured zones depends strongly on the subsidence rates, lithology and the occurrence of heterogeneities such as faults. Modelling results suggest that the presence of vertical fault zones have a dramatic influence on the pore pressure evolution (pressure drain-off), and in consequence on the HC accumulation pattern.

A Model of Secondary Hydrocarbon Migration As a Buoyancy-Driven Separate Phase Flow

A mathematical model of secondary migration is described which permits the prediction of hydrocarbon migration and accumulation patterns in a sedimentary basin, if source rock expulsion rates and geometrical and hydraulic properties of major carrier systems are known through geological time. In this model, secondary migration is treated as buoyancy-driven, segregated flow of hydrocarbons in hydrostatic aquifers. Lateral, updip migration is conceived as a Boussinesq-type, free-surface flow, with source and sink terms representing supply from source rocks and leakage through cap rocks and faults. This permits a two-dimensional, map-view mathematical description of a three-dimensional, time-dependent secondary migration system. A nine-point finite difference approximation has been developed to minimize numerical dispersion, and upstream-weighting is used to obtain stable solutions. Example computations for simple, single carrier bed structures are presented.

Regional Hydrocarbon Generation, Migration, and Accumulation Pattern of Cretaceous Strata, Powder River Basin: ABSTRACT

A cell of abnormally high fluid pressure in the deep part of the Powder River basin is centered in an area where oil-generation-prone source rocks in the Skull Creek (oldest), Mowry, and Niobrara (youngest) formations are presently at their maximum hydrocarbon-volume generation rate. The overpressures are believed to be caused by the high conversion rate of solid kerogen in the source rocks to an increased volume of potentially expellable fluid hydrocarbons. In this area, hydrocarbons appear to be the principal mobile fluid species present in reservoirs within or proximal to the actively generating source rocks. Maximum generation pressures within the source rocks have caused vertical expulsion through a pressure-induced microfracture system and have charged the first available underlying and/or overlying sandstone carrier-reservoir bed. Hydrocarbons generated in the Skull Creek have been expelled downward into the Dakota Sandstone and upward into the Muddy Sandstone. Hydrocarbons generated in the Mowry have been expelled downward into the Muddy or upward into lower Frontier sandstones. Hydrocarbons generated in the Niobrara have been expelled downward into upper Frontier sandstones or upward into the first available overlying sandstone in the Upper Cretaceous. The first chargeable sandstone overlying the Niobrara, in ascending order, may be the (1) Shannon, (2) Sussex, (3) Parkman, (4) Teapo , or (5) Tekla, depending on the east limit of each sandstone with respect to vertical fracture migration through the Cody Shale from the underlying area of mature overpressured Niobrara source rocks. Vertical charge into each of the various carrier-reservoir sandstone units from their related source rock has been followed by a process of dominantly lateral updip migration within the carrier-reservoir bed toward sites of entrapment. Purely updip migration paths have been modified by both stratigraphic complexity and ground-water hydrodynamic flow. Stratigraphic-type traps terminating migration paths predominate on the north flank of the basin. Anticlinal traps predominate on the western and southern flanks. End_of_Article - Last_Page 856------------