Upper Paleozoic total petroleum system and geological model of natural gas enrichment in Ordos Basin, NW China

Based on the analysis of Upper Paleozoic source rocks, source-reservoir-caprock assemblage, and gas accumulation characteristics in the Ordos Basin, the gas accumulation geological model of total petroleum system is determined. Then, taking the Carboniferous Benxi Formation and the Permian Taiyuan Formation and Shanxi Formation as examples, the main controlling factors of gas accumulation and enrichment are discussed, and the gas enrichment models of total petroleum system are established. The results show that the source rocks, faults and tight reservoirs and their mutual coupling relations control the distribution and enrichment of gas. Specifically, the distribution and hydrocarbon generation capacity of source rocks control the enrichment degree and distribution range of retained shale gas and tight gas in the source. The coupling between the hydrocarbon generation capacity of source rocks and the physical properties of tight reservoirs controls the distribution and sweet spot development of near-source tight gas in the basin center. The far-source tight gas in the basin margin is mainly controlled by the distribution of faults, and the distribution of inner-source, near-source and far-source gas is adjusted and reformed by faults. Generally, the Upper Paleozoic gas in the Ordos Basin is recognized in four enrichment models: inner-source coalbed gas and shale gas, inner-source tight sandstone gas, near-source tight gas, and far-source fault-transported gas. In the Ordos Basin, inner-source tight gas and near-source tight gas are the current focuses of exploration, and inner-source coalbed gas and shale gas and far-source gas will be important potential targets in the future.

Formation Mechanism of the High-quality Upper Paleozoic Natural Gas Reservoirs in the Ordos Basin

The upper Paleozoic natural gas reservoirs in the Ordos basin are generally characterized by a large gas-bearing area and low reserve abundance. On such a geological background, there still exist gas-enriched zones, with relatively high outputs, high reserve abundance and stably distributed gas layers. The gas-enriched layers with relatively high permeability （the lower limit permeability is 0.5×10^-3μm^2） are key factors for the enrichment and high output of natural gas. Based on core observation, analytic results of inclusions, and a great deal of drilling data, we proposed the following four mechanisms for the formation of high-quality reservoirs： （1） in the source area the parent rocks are mainly metamorphic rocks and granites, which are favorable to keeping primary porosity; （2） under the condition of low A/S （accommodation/sediment supply） ratios, sandstone complex formed due to multistage fluvial stacking and filling are coarse in grain size with a high degree of sorting, low content of mud and good physical properties; （3） early-stage recharge of hydrocarbons restricted compaction and cementation, and thus are favorable to preservation of primary pores; （4） microfractures caused by the activity of basement faults during the Yanshan Movement stage can not only improve the permeability of tight sandstones, but also afford vertical pathways for hydrocarbon gas migration.

Complex Exploration Techniques for the Low-permeability Lithologic Gas Pool in the Upper Paleozoic of Ordos Basin

The Ordos basin is a stable craton whose late Paleozoic undergoes two sedimentary stages: from the middle- late Carboniferous offshore plain to the Permian continental river and lake delta. Sandstones in delta plain channels, delta-front river mouth bars and tidal channels are well developed. The sandstones are distributed on or between the genetic source rocks, forming good gas source conditions with widespread subtle lithologic gas pools of low porosity, low permeability, low pressure and low abundance. In recent years, a series of experiments has been done, aimed at overcoming difficulties in the exploration of lithologic gas pools. A set of exploration techniques, focusing on geological appraisal, seismic exploration, accurate logging evaluation and interpretation, well testing fracturing, has been developed to guide the exploration into the upper Paleozoic in the basin, leading to the discoveries of four large gas fields: Sulige, Yulin, Wushenqi and Mizhi.

Structural Evolution and Hydrocarbon Potential of the Upper Paleozoic Northern Ordos Basin, North China

The hydrocarbon potential of the Hangjinqi area in the northern Ordos Basin is not well known, compared to the other areas of the basin, despite its substantial petroleum system.Restoration of a depth-converted seismic profile across the Hangjinqi Fault Zone（HFZ） in the eastern Hangjinqi area shows one compression that created anticlinal structures in the Late Triassic, and two extensions in ~Middle Jurassic and Late Early Cretaceous, which were interrupted by inversions in the Late Jurassic–Early Early Cretaceous and Late Cretaceous, respectively.Hydrocarbon generation at the well locations in the Central Ordos Basin（COB） began in the Late Triassic.Basin modeling of Well Zhao-4 suggests that hydrocarbon generation from the Late Carboniferous–Early Permian coal measures of the northern Shanbei Slope peaked in the Early Cretaceous, predating the inversion in the Late Cretaceous.Most source rocks in the Shanbei Slope passed the main gas-migration phase except for the Hangjinqi area source rocks（Well Jin-48）.Hydrocarbons generated from the COB are likely to have migrated northward toward the anticlinal structures and traps along the HFZ because the basin-fill strata are dipping south.Faulting that continued during the extensional phase（Late Early Cretaceous） of the Hangjinqi area probably acted as conduits for the migration of hydrocarbons.Thus, the anticlinal structures and associated traps to the north of the HFZ might have trapped hydrocarbons that were charged from the Late Carboniferous–Early Permian coal measures in the COB since the Middle Jurassic.

Characteristics and accumulation mechanism of tight sandstone gas reservoirs in the Upper Paleozoic, northern Ordos Basin, China

The Ordos Basin is a significant petroliferous basin in the central part of China.The Carboniferous and Permian deposits of transitional and continental facies are the main gas-bearing layers in the north part of the basin.The Carboniferous and Permian natural gas reservoirs in the northern Ordos Basin are mainly tight sandstone reservoirs with low porosity and low permeability,developing lots of ＂sweet spots＂ with comparatively high porosity and permeability.The tight sandstones in the study area are gas-bearing,and the sweet spots are rich in gas.Sweet spots and tight sandstones are connected rather than being separated by an interface seal.Sweet spot sand bodies are vertically and horizontally overlapped,forming a large gas reservoir group.In fact,a reservoir formed by a single sweet spot sand body is an open gas accumulation.In the gentle dipping geological setting and with the source rocks directly beneath the tight reservoirs over a large area,the balance between gas charging into tight reservoirs from source rocks and gas loss from tight reservoirs through caprock is the key of gas accumulation in tight sandstones.Both the non-Darcy flow charging driven by source-reservoir excess pressure difference and the diffusion flow charging driven by source-reservoir gas concentration difference play an important role in gas accumulation.The results of mathematical modeling indicate that the gas accumulation cannot be formed by just one of the above mechanisms.The diffusion of gas from source rocks to reservoirs is a significant mechanism of tight sandstone gas accumulation.

The First Report of Thermochemical Sulfate Reduction Reaction in the Upper Paleozoic Carbonate Rocks of Southeastern Ordos Basin

Thermochemical sulfate reduction （TSR） is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and CO2. TSR has been studied in many sedimentary basins such as China＇s Sichuan and Tarim basins because it has a profound impact on the commercial viability of petroleum resources, with HzS typically being undesirable.

Gas accumulation conditions and key technologies for exploration & development of Sulige gasfield

Up to now,the Sulige area in Ordos Basin has the favorable exploration area of 55×10^(3) km^(2),the total reserve of natural gas of nearly 6×10^(12) m^(3) and the proven reserve(including basic proven reserve)of 4.77×10^(12) m^(3),where the annual production of natural gas reaches 23×10^(9) m^(3),and the Sulige gasfield is the largest onshore natural gas field in China.The pay zone of the Sulige gasfield mainly is Member 8 of Shihezi Formation and Member 1 of Shanxi Formation of Permian which belong to the typical tight sandstone gas reservoir.The coal measure strata in Carboniferous Benxi Formation,Permian Taiyuan Formation and Shanxi Formation provide abundant gas sources for the Gulige gas reservoirs.An open-flow sedimentary model of lacustrine delta is developed,the gentle bottom,sand supply from multisource,strong hydrodynamic force and multi-period superposition control the distribution of largearea reservoir sand body.Lithology of the reservoir is the sandstone of the fluvial-delta facies,the physical property is poor and the heterogeneity is strong,the average porosity ranges from 4%to 12%and the average permeability varies from 0.01 to 1 mD.The gas reservoir is characterized by wide hydrocarbon generation,pervasive hydrocarbon charging,short-range migration and massive accumulation.The pressure coefficient of the gas reservoir ranges from 0.62 to 0.90,indicating the low-pressure gas reservoir,and the single-well yield is low.Full digital seismic technique in the desert area,nonlongitudinal seismic technique in the loess plateau,accurate logging evaluation technique,tight sand reservoir stimulation technology and horizontal well development technology are key technologies for exploration and development of Sulige gasfield.

Petrography and tectonic provenance of the Permian Tunas Formation:Implications on the paleotectonic setting during the ClaromecóForeland Basin evolution,southwestern Gondwana margin,Argentina

The Claromec o Basin is located at the south-western sector of the Buenos Aires province,Argentina.This basin is considered a foreland basin closely related to the evolution of the southwestern Gondwana margin.This contribution focuses on the provenance analysis of the Tunas Formation(Permian,Pillahuinc o Group),which represents the last filling stage for the Claromec o Foreland Basin.Petrographic and tectonic provenance analyses were performed in sandstones recovered from subsurface(PANG 0001 and PANG0003 exploration wells)and outcrops located close to the basin center(Gonzales Chaves locality).In the subsurface,the analyzed succession is composed of medium-to fine-grained sandstones interbedded with tuffs,mudrocks,carbonaceous mudrocks and coal beds.In outcrops,the succession is dominated by mediumto fine-grained sandstones interbedded with siltstones.Modal composition patterns are distributed into the recycled orogen and transitionally recycled to mixed fields.Petrographic analyses,in addition to provenance and sedimentological studies,confirm that sedimentary material was derived from a mixed source,which largely comes from the Sierras Australes fold and thrust belt,located towards the W-SW,where the sedimentary succession is interbedded with volcanic material.The Tunas Formation shows clear differences in its modal composition,paleocurrent direction and paleoenvironmental conditions with respect to the underlying units of the Pillahuinc o Group(Sauce Grande,Piedra Azul and Bonete formations).Source areas changed from cratonic to mixed fold belt/arc-derived material,suggesting variations in the Claromec o Basin configuration during the Late Paleozoic.Changes in the paleotectonic scenario during the deposition of the Tunas Formation have been interpreted as a consequence of a compressive post-collisional deformation event,the product of adjustment,accommodation and translation of terrains towards the equator during the Permian-Triassic to form Pangea.