The sources of the Paleozoic petroleum systems in the Tarim Basin as revealed by geochemistry of oils and extracts from ultra-deep reservoirs

The oils of Shuntuoguole low uplift of Tarim Basin in western China have high contents of saturated hydrocarbons and a high loss of light hydrocarbon components because of volatilization. These oils are characterized by low abundance of hopanes and high abundance of C28–30 tricyclic terpanes and dibenzothiophenes and relative high abundance of C20–23 terpanes, C21–22 5α-steranes, and diasteranes in the oils. Quantitative analysis of carbon isotope and biomarkers show that the oils in the Ordovician carbonate rocks are sourced by deep Cambrian and Ordovician marine shales and marls. Geochemistry of the oils indicates that the reservoir has undergone multistage accumulations, with the late-stage oils obscuring traces of biodegradation of paleo-oils charged in the early stage. The concentration of methyl adamantanes and steranes demonstrates that oils occurred in an early stage of intense oil-to-gas cracking. Most of the gases are wet, with dryness lower than 90%, and geochemical analysis suggests that they are mainly from thermal degradation of kerogen. This study suggests that in addition to oils sourced by deeper source rocks in the Shuntuoguole low uplift in situ, there are oils that migrated from the Manjiaer depression into the Shuntuoguole low uplift.

The oil source of the Lower Paleozoic petroliferous system in the Tarim Basin is still controversial. Based on the detailed analysis of the molecular isotopic geochemistry of the crude oil from well Yubei 1 in the Yubei area of the Tarim Basin, the oil source of the Lower Paleozoic petroliferous system in this area was studied by inversion ideas. The crude oil from well Yubei 1 was the superimposed product of two phases of accumulation, and the oil sources of the two phases were the products of the same set of Cambrian source rocks during the peak period of oil generation. The burial depth of Cambrian source strata in different areas could be the main factor leading to multiple phases of hydrocarbon charging. Crude oil accumulated in the early stage experienced severe biodegradation due to strata uplift and erosion, while oil in the late stage was well preserved. Although the biomarker assemblages and molecular isotopic compositions in the Yubei 1 oil can be generally correlated with oils from the Lower Paleozoic petroleum system in the central Tarim Basin, the distributions of aryl isoprenoids suggested that the depositional environment of the Cambrian source rock in the southern Tarim Basin could be different from that in the central Tarim Basin, which worth a further in-depth study.

For major ultra-deep oilwells in Tarim basin, hydraulic fracturing operation are challenged by the actual vertical depth in excess of 6000 m and the temperature at bottom hole over 160 °C. The long injection path from ground to target formation generates considerable friction, resulting in extremely high ground pressure, which usually exceed the limit of operation equipment. Moreover, traditional crosslinked fracture fluids always lost their viscosity and sand-carrying ability at high temperatures. Consequently, it is meaningful to develop and research novel fracturing fluids for hydraulic stimulation in high-temperature, ultra-deep reservoirs. Nano-composite technology has been proven to be a potential solution to some challenges associated with crosslinked fracture fluid systems. In this work, a kind of low-cost molybdenum disulfide (MoS2) nanosheet was firstly synthesized by hydrothermal chemical method. Afterwards, the MoS2 nanosheets were mixed with polyacrylamide solution with specific molecular weight by ultrasonic dispersion, and a certain amount of organic zirconium crosslinker was added to prepare the nano-composite crosslinked fracturing fluid. Finally, a series of indoor evaluation tests were performed to compare the performance of the nano-composite crosslinked system with the similar crosslinked fluid without adding nanosheets, such as rheology properties, drag reduction efficiency, proppant suspension and retained conductivity. The obtained experimental results have shown that the thermal stability of the nano-composite crosslinked system is much superior to that of a comparable fluid lacking the MoS2 nanosheets. The introduction of nanomaterial allows the novel fracturing fluid to operate at greatly lower polymer concentrations (0.2%-0.3%) compared with current commercial fluid systems (0.4%-0.5%) designed for 180 ℃ reservoirs. The retained apparent viscosity can be maintained above 75 mPa·s after shearing 120 mins at 180 ℃. Rheological studies have shown that the novel system has superior crosslinking properties, and the crosslinking time can be controlled within 4-10 minutes. In addition, this novel nano-composite crosslinked fracturing fluid has enough sand-carrying viscosity under high-temperature conditions, and allows for efficient cleanup by use of an oxidizer-type breaker. Little or no polymer residue and efficient cleanup are contributing to lower reservoir damage, better fluid conductivity, and improved well production. Newly proposed nano-composite crosslinked fracturing fluid provides a new solution for fracturing stimulation of ultra-deep high-temperature reservoirs in the Tarim Basin, and hence improving the oil recovery.

The Tahe oil field is the largest Paleozoic petroleum accumulation in China, with a multistage petroleum-charge history. Ordovician carbonate reservoirs in this field are investigated in this work based on oil geochemistry, fluid inclusion analyses, and in situ U-Pb dating of calcite cements. Four charge episodes are identified based on the following: (1) the similar origin but different maturation histories of sampled crude oils; (2) the four distinct fluorescence-color and fluorescence-spectra parameters revealed by oil inclusions; and (3) the distinct in situ U-Pb ages obtained, which indicate that three stages of calcite cements associated with primary oil inclusions with different fluorescence colors occurred at 459.4 ± 7.5 to 457.6 ± 6.2 Ma, 310.0 ± 3.6 to 304.6 ± 5.5 Ma, and 220.5 ± 7.3 to 215.7 ± 2.6 Ma, with uncertainties of calculated ages reported at the 2σ level. In addition, there are different fluorescence-color and color-spectra parameters between primary and secondary oil inclusions when considering the three stages of calcite cements. In addition, four oil-charge episodes are identified at ca. 445–452.1 Ma, 295.6–311.5 Ma, 215–224.8 Ma, and 108.5–116.6 Ma based on the lower homogenization temperatures (Th) of aqueous inclusions coeval with the oil inclusions. These four episodes relate to the Caledonian, Hercynian, Indosinian, and Yanshanian Orogenies, respectively. This work is important because it suggests that fault reactivation during the main tectonic episodes generated the main pathways for oil in the Ordovician carbonate reservoirs of northwestern China. It reveals that the lower Th of aqueous inclusions coeval with oil inclusions, when combined with the U-Pb dating of calcite cements, can be used to determine the timings of oil charge in the Tahe oil field, not only in China but also in the deeply buried carbonate reservoirs hydrocarbon systems worldwide.

Fracture effectiveness evaluation in ultra-deep reservoirs based on geomechanical method, Kuqa Depression, Tarim Basin, NW China

With the rapid development of the petroleum industry, oil and gas exploration gradually changes from conventional shallow and middle-depth reservoirs to deep and ultradeep reservoirs. In western China, especially in the Tarim Basin, the deep Ordovician and ultradeep Cambrian carbonate is the most important hydrocarbon reservoir. But because of complex near-surface conditions and complicated subsurface structures, high-precision seismic imaging for such deep and ultradeep reservoirs is still challenging with the state-of-art migration methods. One of the critical factors is that the reflections from deep and ultradeep strata cannot be coherently stacked in the migration because of accumulative traveltime errors caused by an inaccurate velocity model. To mitigate this issue, we present a coherent-stack-based least-squares migration (LSM) approach to improve the imaging quality for deep and ultradeep structures. Unlike traditional LSM that uses the stacked gradient during iterations, the proposed method updates the reflectivity model in the subsurface half-opening angle domain and then applies a coherent stacking to implement constructive summation for angle-domain common-image gathers. The new LSM scheme enables us to reduce the artifacts caused by an inaccurate velocity model and produces high-quality images for deep and ultradeep strata. Two models with typical steep-dipping faults, overthrust folds, and fault-karst carbonate reservoirs are designed to test the feasibility of the proposed method, and a field data set from a land survey is used to demonstrate its adaptability for low signal-to-noise ratio data.

Biomarker geochemistry of crude oils and Lower Paleozoic source rocks in the Tarim Basin, western China: An oil-source rock correlation study

The reservoir depth of T-Sh oilfield in Tarim Basin is more than 7500 m, which is a typical deep carbonate fault-controlled reservoir. The S-1 and S-5 fault zones experienced multi-stage tectonic movements and developed complex fault-fracture systems. Based on geometry and dynamics, the evolution characteristics of faults are analyzed; the permeability of strike-slip faults in the middle and lower Ordovician carbonate strata drilled through in S-5 fault zone is studied by comprehensively using core observation, imaging logging, 3D seismic data, and drilling historical data, taking wells F-1 and F-10 as examples. It is found that the fault-fracture system is the main reservoir space and fluid migration channel in the reservoir. A large mud loss will occur when drilling high permeability faults. High production can be obtained after conventional well completion, otherwise, it is difficult to get production. In this paper, slip tendency coefficient is used to quantitatively characterize the permeability of fractures in T-SH ultra-deep reservoir. Based on the one-dimensional geomechanical model and three-dimensional geological structure model of typical wells, the slip tendency coefficients of different parts of the fault-fracture system are calculated using finite element numerical simulation method. Compared with the historical data of drilling in S-1 and S-5 fault zones, it is found that the slip tendency coefficient is positively correlated with mud loss. The results show that the critical slip tendency coefficient of the S-5 fault zone is 0.3, and that of the S-1 fault zone is 0.2. This study provides a new idea and method for the prediction of geological desserts and well trajectory design in the T-Sh reservoir.

To clarify the genetic mechanism for phase change of the hydrocarbon in the ultra-deep reservoirs, a case study from the Ordovician hydrocarbon in the Halahatang–Shunbei area (HSA), Tarim Basin, NW China, was conducted. The results show that the Ordovician reservoirs in the HSA are characterized as multi-phase reservoirs with a lateral co-existence of condensates, volatile-oil reservoirs, normal oil reservoirs, and heavy oil reservoirs. From north to south, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon in different blocks of the HSA, showing an increasing trend in GOR, dryness coefficients, methane contents, methane carbon isotope values, and ethane carbon isotope values, while a decreasing trend in oil densities and wax contents. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed and the kerogen-cracking gas is dominated in the HSA, the regular variations of the hydrocarbon phases and geochemical characteristics can be interpreted as records of biodegradation and multistage oil–gas filling rather than controlled by the source rock organofacies, oil cracking, and gas invasion. The formation mechanism of the Ordovician multi-phase reservoirs in the HSA suggests that the deep strata of the Tarim Basin hold potential for the exploration of natural gas resources.

Thermal maturity, source characteristics, and migration directions for the Ordovician oil in the Central Tabei Uplift, Tarim Basin: Insight from biomarker geochemistry

The high density and wide azimuth seismic acquisition has become a routine seismic exploration technology in the Tazhong desert area of Tarim Basin. The acquisition of broadband signals depends on excitation, receiving, geological conditions and subsequent seismic data processing methods. Broadband acquisition has significant practical influence on obtaining wideband signals, especially low frequency signal. With higher requirement in development, broadband seismic acquisition has remarkable advantages at low frequency end, and has desired effects in the development of ultra-deep fractured-vuggy reservoirs in the Tazhong desert area in. First, the enhancement of low-frequency signals improves the recognition accuracy of effective reflection in deep and ultra-deep layers, thus, consolidating the description foundation of fault-dissolved reservoir. Secondly, the low-frequency response is sensitive to the scale of fractured-vuggy bodies. The seismic reflection energy of fractured-vuggy bodies with a certain scale is stronger than conventional seismic, which provides a reference basis for evaluating the input and output benefits of well location deployment. The application shows that broadband receiving seismic data can be used to effectively guide the implementation of heterogeneous fractured-cavern reservoirs’ development plan, and can be popularized in the exploration and development of ultra-deep fractured-cavern reservoirs in desert.

The hydrocarbon phase state of deep to ultra-deep reservoirs in the Tarim and Sichuan basins has been of great interest in oil and gas exploration. Based on a combination of molecular dynamics simulation, gold-tube pyrolysis experiments, and geological-geochemical theory, this study discusses the mechanisms governing the stability of oils in deep reservoirs from the perspectives of their reservoir accumulation histories and chemical reactions. Generally, the reason for the existence of liquid oil in the Tarim Basin is widely considered to be only controlled by external geological conditions, mainly including low geothermal gradient, absence of thermal events, low maximum reservoir temperatures, and late hydrocarbon generation process. However, this study firstly proposed that the chemical composition of oil is an internal factor for its thermal stability. The simulation results reveal that the polycondensation reactions of asphaltene will release hydrogen atoms, which can provide a necessary hydrogen source for cracking of liquid chain hydrocarbons. It means that the presence of asphaltene components can promote the cracking of chain hydrocarbons and generate methane. The normal mature oil in the Sichuan Basin generally has higher contents of asphaltenes than that of the high-mature light oil of the Tarim Basin, so more hydrogen has historically been available for the cracking of oil to gas. By looking at the accumulation histories and chemical compositions of the crude oils, this study first explains the stable long-term storage of liquid hydrocarbons in the Tarim Basin, providing important guidance for future deep to ultra-deep oil and gas exploration.

Fracturing in ultra-deep wells is one of the important means of improving ultra deep reservoir conditions and making it reach commercial development value. There are more difficulties in the optimization design and the safety of operation due to factors such as higher formation temperature, higher pumping pressure, and higher investment risk. This paper discusses the recent development of fracturing technology in ultra deep wells in China:history of fracturing in ultra-deep wells;successful fracturing experience in ultra-deep well in DH Oil Field, Tarim, China. Drawing the lesson and experience of fracturing in ultra-deep well in early years, combining with the research and practice of stimulation in ultra deep low-permeability reservoir in Tarim Basin(5800 to 5900 m in depth,140°C), aiming at the properties of fracturing for ultra deep formation, applying fully 3D fracturing design software, a methodology of predicating the accurate pumping pressure, understanding the state of fracture extension, optimizing the fracturing fluid characteristics has been used to optimize the fracturing designs. In addition, advanced operational equipment and matching techniques have been applied to DH Oil Field. Significant stimulation results in production and injection wells were obtained with the applications of the fracturing technology in DH ultra deep oil field. The water injection profile testing results showed that the post-fracture water injection rate of the 4 fractured wells was at the range of 140 to 170 m3/d, the operational success rate of the four fractured wells was 100%, and the effective rate of water injectivity was 100 %. Field practice shows that China not only has the capability of fracturing in ultra deep wells but also obtains good post-fracture response at about the depth of 6000m at present. Introduction With the extension of exploration and development of hydrocarbon into deeper formations, hydraulic fracturing in these formations has become more and more important. The properties of fracturing for ultra deep wells include that:deep formation(>4200m);high temperature (>120°C);high fracture pressure>100MPa); andhigh operational pumping pressure(>60MPa). Therefore, as compared with conventional fracturing, fracturing in ultra-deep wells add more difficulties to the optimization design and the safety of operation due to the above factors. In addition, the investment risk is raised. From the first fracturing treatment in Yumen Oil Field, China in 1955, not only the quantity and scale of fracturing operation, but also the depth of fractured wells have been deepen with the extension of exploration and development of hydrocarbon. The deepest ultra deep well in China's fracturing history at that time—Wugu 1 well (4827.41 to 4911.0m in depth), Huabei Oil Field. Had been fractured successfully on Feb.1,1989. The main area of fracturing for ultra deep well in China had been switched from east to west with exploration and development of the west Tarim Basin at the end of 1980's, the depth of ultra deep fractured well was beyond 5000m, too. However, because of the limitation of fracturing equipment and technology for ultra deep well, the operational success rate was only 50% from 1989 to 1993, as shown in Table 1. Since 1995, considering the under-injected conditions of water injection well, the following aspects have been deeply performed to improve the ultra deep well fracturing technology. That is the understand of formation parameters, the optimization of 3D design, the selection of operational equipment and down hole tools, the optimization and selection of fracturing material and fluid formula, and the control of fracturing fluid quality on the spot. The operational success rate was 100%, as shown in Table 4. It has eventually changed the passive situation of the formerly ultra deep well fracturing—"lower operational success rate and easy sand screen-out".

Kelasu structural belt in Tarim Basin has a large reservoir burial depth and complex geological conditions, and challenges such as ultra-deep, high temperature, high pressure, and high stress lead to big problems related to well control safety and project quality. To solve these key technical problems that set barriers to the process of exploration and development, a set of drilling technology processes via geology-engineering integration is established with geomechanics as the bridge. And an integrated key drilling engineering technology for the safe speed-up of ultra-deep wells was formed, integrating well location optimization, well trajectory optimization, stratum pressure prediction before drilling, stratum drillability evaluation, and bit and speed-up tool design and optimization. Combined with seismic data, logging data, structural characteristics, and lithology distribution characteristics, the rock mechanics data volume related to the three-dimensional drilling resistance characteristics of the block was established for the first time, and the vertical and horizontal heterogeneity was quantitatively characterized, which provided a basis for bit design, improvement, and optimization. During the process of drilling, the geomechanical model shall be corrected in time according to the actual drilling information, and the drilling “three pressures” data shall be updated in real-time to support the dynamic adjustment of drilling parameters. Through field practice, the average drilling complexity rate was reduced from 18% to 4.6%, and the drilling cycle at 8500 m depth was reduced from 326 days to 257 days, which were significantly better than those of the vertical wells deployed in the early stage without considering geology-engineering integration.

Halahatang oilfield in Tarim Basin is a typical ultra deep fractured vuggy carbonate reservoir, which is the main production area of crude oil in Tarim Basin. The wellbore instability is serious in Triassic and lower strata (the borehole enlargement rate is 22% - 31%). Due to the complex rock characteristics, the drilling cycle for single well is long (124 days on average), and it is difficult to speed up. Based on the log interpretation curves, the calculation methods of pore pressure, collapse pressure and fracture pressure profiles established, and the spatial distribution characteristics of three pressures in this area were analysed; The rock drillability, rock hardness, compressive strength and abrasiveness of the roller bit and PDC bit single well measured by laboratory tests. Based on the statistical regression method, the relationship between the above drilling rock mechanical parameters and logging acoustic time difference and the formation drillability profile single were studied. The research results guide the drilling design optimization and operation of 15 wells in Ha 8 and Ha 12 blocks, The average drilling cycle is reduced by 21% (from 124 days to 98 days), and the hole enlargement rate is reduced from 24% to less than 10%.