Jin Su

The formation mechanism of high dibenzothiophene series concentration in Paleozoic crude oils from Tazhong area, Tarim Basin, China

Dibenzothiophene series is one of the most important compositions in crude oil, which generated under multiple geological and geochemical processes. The relationships between dibenzothiophene series and other biomarkers (C29 αα sterane 20R, and C28 triaromatic sterane 20R), combined with the research of thermochemical sulfate reduction (TSR), biodegradation, geological ages, and oil source in Paleozoic oil in Tazhong area, indicated that there are three control factors for high concentration of dibenzothiophene series. First, both middle-upper Ordovician and lower Ordovician-Cambrian source rocks are marine carbonate sedimentary rocks which could produce abundant organic sulfur compounds including dibenzothiophene series. Second, biodegradation could cause the enrichment of organic sulfur compounds. In addition, sulfate reducing bacteria was able to transfer hydrocarbons and S in SO42− in oil-bearing reservoir water into organic sulfur compound. It might be the main path to produce dibenzothiophene series. Third, sulfur compounds might have been formed by TSR, because S of SO42− in oil-bearing reservoir water could also be transferred to H2S and organic sulfur compounds under high temperature, which might result in increase of dibenzothiophene series in crude oil.

Characteristics and Accumulation Mechanism of Quasi-Layered Ordovician Carbonate Reservoirs in the Tazhong Area, Tarim Basin

Weathered crust reservoirs of marine carbonate rocks and large-area quasi-layered carbonate reservoirs controlled by massive unconformity surfaces have become an important play for exploration of marine hydrocarbons across China. In the Tazhong area, oil and gas originating from the marine facies of the Ordovician Yingshan formation, present large-scale integrated continuous accumulation along massive structures. In particular, they are accumulated continuously in layered form in carbonate fracture-cavity reservoir system, and not controlled by tertiary structural belts. Moreover, the Yingshan reservoirs are complex in types, comprising cross distribution of high gas/oil ratio (GOR) condensate gas reservoirs, volatile oil reservoirs and normal oil reservoirs, and large burial depth span and high heterogeneity of quasi-layered reservoirs. The complexity in diagenesis and accumulation lead to the unique distribution and enrichment of carbonate reservoirs in the Tazhong area. The analysis of blow-down and leakage depths, heterogeneity, and physical properties of pores, caves and fractures of the Yingshan formation carbonate reservoirs indicates that the interstratal karsts have superimposed with faults and fractures to form fracture zones, so that the reservoir bodies present integrated continuous distribution characteristics spatially, providing favorable conditions for quasi-layered continuous enrichment of oil and gas. During late periods, a massive amount of natural gas had been filled along the gas-source faults. Based on the variations of relative contents of light hydrocarbons and aromatic naphthalene homologs in crude oil, it is found that the quasi-layered continuous carbonate reservoirs in the Tazhong area are different in terms of the properties of oil and gas and the types of oil reservoirs as a result of the formation of high GOR high-yield condensate gas reservoirs near the oil/gas-source faults due to gas washing, migration and fractionation. Late gas charge and transformation process acting on earlier oil reservoirs is an important geochemical action in the formation mechanism of carbonate reservoirs in the Tazhong area.

The Geological Characteristics of Reservoirs and Major Controlling Factors of Hydrocarbon Accumulation in the Ordovician of Tazhong Area, Tarim Basin

The ancient deep-buried marine carbonate reservoir takes on strong heterogeneity and fluid complexity, and features large acreage and low abundance. For the ancient deep carbonate exploration in China now, it is challenging to find accumulating positions with high abundance and deploy high-efficiency wells. Based on the E&D practices of Ordovician carbonate reservoir and the high-efficiency well correlation in the Tazhong area, Tarim Basin, it is found that the high-efficiency wells distribute along the peaks of third-order structure zone and the relative high parts of platform, about 0.31.5km away from the strike-slip fault, and longitudinally, mainly in large unfilled Ordovician fracture-cavity reservoirs in the lower O1y1, the lower O1y2 and the O3l3. The large scale unfilled fracture-cave system (as favorable reservoir space), the strike-slip fault (as pathway for efficient hydrocarbon charge in late stage) and the sufficient hydrocarbons (charged intensively in multi-stages) contribute to the high abundance hydrocarbon enrichment in the Ordovician karst carbonate reservoir across the Tazhong area. This research recommends to use precise structure interpretation and palaeogeomorphic and palaeotectonic reconstruction to identify the gas source faults and favorable charging position, use precise fracture-cavity description to assess the cavity quantitatively, and use seismic wave impedance inversion and hydrocarbon detection to predict the distribution of large unfilled cavity reservoirs, in order to effectively estimate the high-abundant oil/gas area, locate the high-efficiency well area and improve the E&D performance in ancient deep carbonate rocks.

Geology and hydrocarbon accumulation of the large ultra-deep Rewapu oilfield in Tarim basin, China

Over the past five years, oil and gas exploration to marine facies carbonate rock in China took on a fast-growing tendency, and a lot of large-scale oil and gas fields were discovered in succession. Particularly in the north of Tarim Basin, the large-scale Rewapu oil field was recently discovered in the Ordovician carbonate rock layer system with buried depth of 7,000 meters, manifesting that a significant potential of exploration exists in Tarim Basins deep layer and ultra deep layer of Tarim basin. The results indicate that reservoirs of Rewapu oilfield are low porosity and low permeability layers karst reservoirs, reservoir temperature is around at 160 °C with oil, and the crude oil comes from the upper Ordovician source rock of the Manjaer depression. The hydrocarbon accumulation time is in the Late Permian, and since the Triassic sedimentation, the reservoir is in the continuous process of burying. It belongs to the old oil and gas system. Due to low geothermal gradient and late quick deep burial, oil reservoir has yet to be cracked, so well-preserved old oil reservoir can be discovered at the depth of 7000 m. The Ordovician carbonate reservoir in Rewapu was a fracture-cavity oil reservoir, and the oil and gas distribution and enrichment are controlled not by the local structure, but by the distribution and the development degree of Ordovician carbonate reservoir. The reservoirs of Rewapu block are mainly distributed in the Ordovician Yijianfang formation and at the top Yingshan formation, and the reservoir development degree was controlled by the karstification, the degree of fracture development and the sedimentation facies, concentrating into belts in plane and not controlling by the depth vertically. The overlying strata are limestone of Tumuxiuke formation and marls and limestone of Sangtamu formation in the upper Ordovician. Rewapu oil reservoir features stratified oil and mainly contains normal oil with low viscosity and high wax. Research results show that Ordovician reservoirs in Rewapu block have relatively large depth of burial, but they are still favorable in quality and sit along the path for the oil and gass northward migration. Therefore, this block has immense resources potential and boasts good exploration and development outlook. In this paper, we study Rewapu block oil reservoirs geological features and hydrocarbon accumulation mechanism, which is of important significance for directing the exploration to oil and gas at the deep layer and ultra deep layer of Tarim Basin.

Distribution and implication of adamantane in crude oils in Lunnan area, Tarim basin in China

According to the present research, the cage structure makes adamantane very stable to resist cracking even at a higher temperature, so its concentration will be increased with temperature rising. But adamantane quantitative analysis of oil in Lunnan area shows a poor correlation between its concentration and geotemperature. Instead, it has a good relationship with maturation parameters such as the content of C29ααα 20R sterane and C28 triaromatic steroid 20R as well as Ts/Tm. The reason is that the oil and cracked products generated in deep zone upwards charged in oil layers in low maturity and then turned into mixed oil which had higher concentration of Ts, adamantine etc. So it makes the parameters such as Ts/Tm ratio and adamantine concentration elevate. The incorporation a great amount of low molecular hydrocarbons diluted C29 ααα 20R sterane and C28 triaromatic steroid 20R, which resulted in the parameters of the mixed oil to indicate a higher maturation. Oils with higher adamantane concentration are mainly distributed on Lunnan fault-host zone, Sangtamu fault-host zone and Jilake structure zone. Cracked oil generated in deeper bed migrated along fractures which acted as channels upwards into the reservoir, and so adamantane content of the mixed oil increased. The source of Paleozoic oils in Tarim basin is involved only in two beds: lower Ordovician-Cambrian and middle-upper Ordovician. But according to the data in this paper, there is no obviously difference in adamantane concentration of oils from the two layers.

Geological features and hydrocarbon accumulation in the Xinken oil field, Tarim Basin

Recently, a large-scale Ordovician oil and gas pool has been discovered in Xinken, north of the Tarim Basin, and it has strongly heterogeneous reservoirs and complicated fluid distribution. Through analyzing oil, gas and water samples collected from this area, in combination of field production and testing data, this study reveals that the Xinken oil pool contains dominantly normal oil characterized by low viscosity, low sulphur, and high wax. It has a low content of natural gas, which presents typical characteristics of wet gas and is oil-associated gas. Oil-gas correlation shows that the oil was from the Middle-Upper Ordovician source rocks and the charge and accumulation of oil and gas took place in the Late Hercynian Period. Controlled by paleogeomorphology, sedimentary facies, ancient karstification, faults, and other factors in the Caledonian Period, the Ordovician carbonate reservoirs are dominated by dissolved pores, cavities and fractures. The reservoir body of fractures and cavities is distributed in the shape of strip and laminate along faults and excellent reservoirs are mostly in the range of 70–100 m below the top of the Yijianfang Formation. As a whole, the Xinken oil pool is a fracture-cavity pool controlled by interbedded karst reservoirs and is a complex composed of stacked karst fractures and cavities of various sizes. It contains oil in large areas horizontally, which is quasi-laminated in distribution, and the accumulation of oil and gas is controlled by the distribution and development degree of the Ordovician carbonate reservoirs. The study shows that this area has abundant resources and a great exploration potential for oil and gas.

The Genesis and Prospecting Significance of High-sulfur Gas Condensates in the Deep Dolomite Reservoirs Beneath Gypsum Rocks: A Case Study of the Cambrian Reservoir in Tarim Basin

Comprehensive two-dimensional gas chromatography (GC×GC-TOFMS) was used to detect recently discovered condensates in the Cambrian dolomite reservoir beneath gypsum rocks of Tarim Basin. It was found that the thioadamantane and dibenzothiophene compounds are rich in the dolomite reservoirs, which indicates that the gas condensates have undergone hydrocarbon charging with TSR genesis. Based on the contents of SO42− and Mg2+ in the formation water of the Well ZS-1 reservoirs, the dolomite reservoirs and gypsum caps of intraplatform tidal flat facies sediments have been proven to be an effective geological combination to provide sulfate contact-ion pairs for TSR initiation. Compared with typical high-sulfur gas reservoirs with TSR throughout the world, the TSR extent in the Lower Cambrian dolomite reservoirs of ZS-1 is lower, and the liquid hydrocarbons in the Lower Cambrian of Well ZS-1 at the buried depth close to 7,000 m have not intensively cracked. Therefore, it is speculated that abundant cracking gas may occur in deeper slopes and basins (about 9,000 m) with high content of H2S and CO2. It is also revealed that deep dolomite reservoirs beneath gypsum rocks are the actual geological conditions for the accumulation and preservation of secondary H2S-bearing gas condensates.