Dolomite reservoir differences in characteristics and controlling factors of grainstone shoal facies in Qixia Formation, Sichuan Basin

Exploration of natural gases in the Middle Permian Qixia Formation has achieved a breakthrough in the central and western Sichuan Basin since 2010. This study discusses the quality of dolomite reservoirs and source rocks, trap types and hydrocarbon charging history and proposes that good accumulation conditions are the basis for the large-scale enrichment of natural gas in the Qixia Formation. The high-quality reservoirs in the Qixia Formation are dolomites that primarily occur along platform margins and in the inner-platform shoals surrounding the Caledonian palaeo-uplifts. The Qixia gases originated mainly from the Cambrian and Permian mudstone source rocks. The overlaying Permian source rocks and the underlying Cambrian source rocks are both widely distributed and could provide sufficient gases for dolomite reservoirs. Additionally, the structural or structural-lithologic traps formed by an overlap of the Qixia dolomites and structurally high are favourable for oil and gas accumulation. The Qixia Formation had experienced four episodes of oil and gas charging from the middle-late Triassic to the Early Cretaceous, and traps located in structural-high zones are most favourable for hydrocarbon accumulation. Thus, the Qixia Formation has excellent accumulation conditions for the formation of large-scale gas fields. The concealed structural belts in the footwall of fault No. 1, the Longmenshan–Daxingchang structural belts and the periphery Gaomo–Shehong palaeo-uplift are the key zones for large-scale gas exploration in the Qixia Formation. The Qixia Formation is an important strategic replacement field for natural gas exploration in the Sichuan Basin following the Ediacaran Dengying Formation and the Cambrian Longwangmiao Formation.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

The hydrothermal fluid–carbonate rock reaction is frequently regarded to occur in deep-burial diagenesis, and the hydrothermal dissolution is usually distributed and takes place along the faults. Previous studies have suggested that there was hydrothermal fluid activity locally in the Permian Qixia Formation in Sichuan Basin, likely related to the Emeishan basalt eruption. However, the effect of hydrothermal fluids on the carbonate rocks of the Qixia Formation in the central uplift of Sichuan Basin is still unclear. Based on the characteristics and geochemical parameters of the diagenetic minerals, this study aims to reveal the diagenetic alteration related to the hydrothermal fluid–rock reaction in the Qixia Formation and reestablish the diagenetic evolution by using the timing of diagenetic mineral precipitation. The methods include petrographic observation; trace and rare earth element (REE) analysis; C, O and Sr isotope measurement; fluid inclusion temperature measurement and cathodoluminescence analysis. According to the petrographic characteristics, the dolostones are mainly of crystalline structure, namely fine-medium crystalline dolostone, meso-coarse crystalline dolostone, and coarse crystalline dolostone, with the cathodoluminescence color becoming brighter in that order. The limestones from the Qixia Formation are of the bioclastic limestone type, with no cathodoluminescence color. Compared with dolostones, limestones have higher Sr content, lower Mn content, and heavier oxygen isotopes. With the crystalline size of dolostone becoming coarser, the oxygen isotopes of dolostones tend to become lighter. The meso-coarse crystalline dolostone has the highest Mn content and negative carbon isotope. Both limestones and dolostones have an obvious positive Eu anomaly in the Qixia Formation. However, the REE patterns of fine-medium crystalline dolostones are very different from those of meso-coarse crystalline dolostones. It is credible that there were two periods of hydrothermal fluid charging, with different chemical compositions. The first period of hydrothermal fluids could laterally migrate along the sequence boundary. Fine-medium crystalline dolostones were almost completely distributed below the sequence boundary and were dolomitized during the shallow burial period. As products of the hydrothermal fluid–dolostone reaction, the saddle-shaped dolomites in the meso-coarse crystalline dolostones were the evidence of the second period of hydrothermal fluids. As a result, the dolomitization model was established according to the timing of diagenetic mineral precipitation, which can improve that the geological understanding of the effect of hydrothermal fluid activities on the carbonate rocks in the Qixia Formation.

A comprehensive drilling of wells has been conducted in the Permian Qixia Formation in the central Sichuan Basin, revealing a significant number of dolomite reservoirs. High- and medium-porosity dolomite reservoirs are the main gas-producing reservoirs in the Qixia Formation. Seismic PP-wave data show a “bright spot” for high-porosity dolomite reservoir formations but weak responses for medium-porosity dolomite reservoir formations, which is attributed to the inability of P waves to distinguish between medium-porosity reservoirs and limestone. However, medium-porosity dolomite and limestone have different S-wave velocities. Therefore, in this study, the identification of different-porosity dolomite reservoirs using multi-component seismic data was investigated. A comprehensive analysis of the elastic waves by forward modeling shows that the PS-wave amplitude is more sensitive to medium-porosity dolomite than the PP-wave amplitude. Therefore, medium-porosity dolomite reservoirs can be predicted using the amplitude attributes of the PS-wave, and high-porosity dolomite reservoirs can be characterized using the PP-wave. Meanwhile, the elastic parameter λρ (the product of Lame constant λ and density ρ), which is highly correlated with the dolomite content, can be used as an indicator of dolomite formations. Furthermore, compared to the results of PP-wave inversion, the elastic parameters derived from the joint inversion of PP- and PS-waves exhibited a better correspondence with the well-logging results. The comprehensive use of the seismic amplitude responses of PP- and PS-waves and multi-component seismic joint inversion can effectively predict high- and medium-porosity dolomite reservoirs. The predicted results can support the exploration and development of the Qixia Formation.

Sucrosic dolomite, an important hydrocarbon reservoir, has long been the focus of carbonate sedimentological and reservoir geological studies. This study investigated a kind of heterogeneous sucrosic dolomite in the Lower Permian Qixia Formation of NW Sichuan Basin, which has recently been the location of giant natural gas discoveries. The heterogeneous sucrosic dolomite is characterized by coexistence of porous euhedral dolomite and tight anhedral dolomite, and it is mainly distributed in the platform-marginal shoal facies with a quasi-layered structure. Further geochemical analysis, including C, O, and Sr isotopes as well as rare earth elements, reveals that the euhedral dolomite and anhedral dolomite have similar geochemical properties to the matrix limestone representing coeval seawater, and they were mainly generated from dolomitization by the closed marine-related fluid (left-leaning REE and δPr < 1) in the shallow burial. The difference in crystal morphology, porosity, and permeability between the euhedral dolomite and anhedral dolomite is mainly related to the compositional and textural heterogeneities of the host rocks. Due to the dissolution of meteoric water (relatively flat REE and low Y/Ho) in the early diagenetic stage caused by high frequent exposures, quasi-layered vugs and caves were formed in the grainstones. In the process of shallow burial dolomitization, the loose-filled carbonate sands formed the porous euhedral dolomite due to sufficient space, while the matrix limestone formed the tight anhedral dolomite due to relatively poor porosity and permeability. Accordingly, the paleogeomorphic highland controlled platform-marginal shoal superimposed by meteoric water dissolution in the early diagenetic stage is the main factor for the formation of Qixia Formation reservoirs, while dolomitization is mainly manifested as the inheritance and adjustment of pre-existing pores in the host rock. Therefore, the exploration direction for dolomite reservoirs in the Qixia Formation in the Sichuan Basin should be shifted to the favorable sedimentary facies-controlled reservoir model, which can also be referential for other cases under similar geological setting.

Summary Several sets of thin dolomite reservoirs are developed in the Permian Qixia Formation in the Sichuan Basin, but the reservoir prediction is extremely difficult due to the deep burial and low seismic resolution of the Qixia Formation, which makes the seismic response characteristics of the reservoir unclear. In this paper, we determine the differences of dolomite in multi-wave seismic data from seismic section phenomena. The seismic response characteristics of different wells are categorized and analyzed. Combined petrophysical and orthorectified simulation analysis, it is verified that the advantages of PS wave data in dolomite prediction, and provide ideas for reservoir prediction in the Qixia Formation in central Sichuan.

Deep-ultradeep oil and gas reservoirs in complex structural areas have typically undergone multistage tectonic processes that have a considerable influence on the hydrocarbon composition, isotopic composition, and other geochemical characteristics of petroleum. By assessing the charging period of hydrocarbon accumulation during a particular age, we can determine these changes. We study the characteristics of fluid inclusions developed in the dolomite reservoir in the Middle Permian Qixia Formation in the Shuangyushi structure of the northwestern Sichuan Basin using methods such as inclusion petrography, microbeam fluorescence spectroscopy, and inclusion homogenization temperature. Furthermore, the main peak wavelength of the fluorescence spectrum, the red-green ratio/chromatic parameter, Q650/500, as well as the chromatic parameter corresponding to the emission flux at 535 nm (QF535), and their correlations with the homogenization temperature distribution are used to identify the charging period of hydrocarbons in the Qixia Formation. The results indicate that different types of fluid inclusions are developed in the Qixia Formation. The fluorescence of hydrocarbon inclusions is mainly blue, with a small amount of yellow-green and orange-yellow. The λmax of orange-yellow, yellow-green, and blue fluorescence of the hydrocarbon inclusions ranges between 562 and 579 nm, 497 and 548 nm, and 447 and 497 nm, respectively. The distribution of the homogenization temperatures during the same period ranges from 100°C to 110°C, 120°C to 140°C, and 160°C to 170°C, respectively. By combining the burial and thermal evolution histories of the Qixia Formation, we conclude that the Qixia Formation has experienced three stages of oil and gas charging, with the time of 204, 185, and 158 Ma. This method is of great significance in determining the date and duration of oil and gas accumulation and provides technical support for the recovery of ultra-deep hydrocarbon accumulations in complex structural areas.

Characteristics and exploration prospects of Middle Permian reservoirs in the Sichuan Basin

A thorough investigation of the characteristics and formation mechanism of dolomite is greatly significant in assessing the validity of dolomitization theory and dolomite reservoirs. Extensive development of dolomites has been found in the organic bank of the Middle Permian Qixia Formation in the northwest of the Sichuan Basin. For that reason, field profile and drilling core samples were collected from the common dolomitic “leopard-spot” limestones in the Middle Permian Qixia Formation in the study area and observed in this work. The diagenetic fluid and formation factors of dolomites were analyzed through major elements, microelements, carbon, oxygen and strontium (Sr) isotope, and fluid inclusion, as well as order degree of dolomite. From the extracted outcomes, valuable insights can be derived. More specifically, in the study area, dolomitic “leopard-spot” limestones were mainly developed in the upper part of Section II in the Qixia Formation. Interestingly, the vertical distribution plays a dominant role since it was transited gradually and gently to the thick-layered lumpy crystalline dolomites downward. The dolomitic “leopard-spots” are mainly composed of dolosiltite-fine crystalline dolomites with residual fabrics, and the base limestones were mainly formed by biomicrites (debris). In addition, the average δ13C of dolomitic “leopard-spot” and base limestones was 3.06‰ and 3.31‰, respectively, whereas their average δ18O was −4.29‰ and −3.16‰ and the 87Sr/86Sr (0.70734 in average) was higher than those of the contemporaneous seawater and mantle provenance range. Moreover, the contents of the major elements in dolomitic “leopard-spot” and base limestones were basically consistent. The uniform temperature of inclusion ranges between 70°C and 115°C. According to the geochemical data and the previously reported studies on the regional paleogeotemperature environment, the diagenetic fluid of the dolomitic “leopard-spot” limestones in the Qixia Formation in the northwest Sichuan Basin was mainly attributed to contemporaneous sea flows. However, influenced by the local high-temperature environment, the terrestrial atmosphere, and the fresh water, it is speculated that the dolomitic “leopard-spot” limestones in the study area might belong to dolomitization involving fresh water from the penecontemporaneous stage to the early shallow burial stage.

Reservoirs in the dolomites of the Middle Permian Qixia Formation in the Sichuan Basin are currently important oil and gas exploration objects in China. However, the questions concerning the sources of the dolomitized fluids and the control factors of the Qixia hydrothermal dolomites remain unclear. In this study, the original hydrothermal dolomites (the replacement dolomites (RDs) and saddle dolomites (SDs)) from the Qixia Formation in the southwestern Sichuan Basin (the PR1 well and Baoxing section) were mainly examined using novel in situ carbonate U-Pb dating with clumped isotopes (∆47). Our results show that the U-Pb ages of the latest SDs from the PR1 well (located in the middle zone of the Emeishan large igneous province (ELIP) and distanced from the Orogenic Belt of Longmenshan) are 257.9–251.0 Ma, coincident with the period of main activity of the ELIP. Combined with the previous U-Pb dating, we propose that the high-temperature T∆47 (82.2–108.4 °C and 127.5–205.9 °C) recorded for SDs from the PR1 well and Baoxing section may have responded to ELIP activity and Longmenshan orogeny activity, respectively. In addition, in the entire southwestern Sichuan Basin, the RDs and SDs yield similar δ13C and δ18O values, indicating that the dolomites were formed by hydrothermal fluids of similar sources, with marine hydrothermal fluids being a highly possible source. Finally, this study proposes a new hydrothermal dolomite genesis model for the Qixia Formation, emphasizing that the formation of hydrothermal dolomites mainly depends on the proximity to tectonic thermal events in space and time.

Source and accumulation processes of natural gases in the Qixia Formation in the Northwestern Sichuan Basin, SW China

Using clumped isotopes to determine the origin of the Middle Permian Qixia Formation dolostone, NW Sichuan Basin, China

Ultra-deep carbonate reservoirs in the Permian Qixia Formation of the Sichuan Basin, particularly those exceeding 7000m in depth, have emerged as a significant focus for exploration and development. However, production capacities between adjacent wells can vary by up to two orders of magnitude due to the multi-scale heterogeneity of pore connectivity, which poses challenges for accurately predicting well performance. This study utilizes thin-section analysis, high-pressure mercury injection (HPMI), scanning electron microscopy (SEM), and other techniques to investigate pore connectivity in ultra-deep reservoirs. It also explores methods for evaluating the connectivity of multi-scale pore networks in the Qixia Formation. The analysis reveals distinct permeability contribution patterns and connectivity characteristics across different reservoir types. Results indicate that the carbonate reservoirs in the Qixia Formation are predominantly composed of dolomite, with intercrystalline pores, dissolution pores, and fractures constituting the primary pore types. The pore-throat size distribution exhibits significant heterogeneity, as evidenced by a multi-peak distribution curve. Approximately 25% of the reservoirs contain well-connected pores, and a threshold radius (r25) is identified as a key parameter for assessing connectivity. Overall, pore connectivity within the reservoirs is limited, with fractures playing a critical role in linking isolated pore spaces. This study introduces the parameter SHgf, which quantitatively evaluates the connectivity of multi-scale pore networks and distinguishes the abundance of fractures within the reservoir using a boundary value of 7%. By analyzing fluid seepage patterns, a permeability contribution model for the four identified reservoir types is established, providing a robust framework for assessing reservoir connectivity. These findings offer valuable insights for predicting production capacities and optimizing development strategies in ultra-deep carbonate reservoirs.

The study area is Longnvsi (LNS) in the central Sichuan Basin, Western China, and the target layer is the Qixia Formation of Lower Permian, with characteristic of thin dolomite reservoirs, low porosity as well as rapid changes and discontinuity of reservoirs in the lateral direction. It’s difficult to predict reservoirs only by PP-wave data. With the development of multi-component exploration, multi-wave seismic data analysis enhances confidence in interpretation by providing PS-wave data for imaging the subsurface. PS seismic data exhibit significant changes in amplitude and character of seismic events that may not be observed on PP seismic data. When PP and PS data are analyzed together, multi-wave seismic exploration has potential and advantages in fluid and lithology prediction. In this paper, logging analysis is carried out for the target layer, which shows that the reservoir response characteristics of P-wave velocity and S-wave velocity are different, then these sensitive elastic parameters of the reservoir can be determined. By comparing the seismic response characteristics of PP-wave and PS-wave, the reservoir seismic amplitude is also different. The difference between P-wave and S-wave in logging and seism is the basis for PP+PS joint inversion. We tried to perform PP+PS joint pre-stack inversion under the theory of P-wave and PS-wave elastic impedance inversion. In this process, we illustrate some key technologies, such as multi-wave calibration, multi-wave wavelet estimation and PP-PS seismic matching. Compared with PP seismic pre-inversion alone, the PP+PS joint inversion increases the constraint conditions, reduces the inversion ambiguity, gives markedly superior estimates of the P-wave and S-wave impedance and Vp/Vs, and improves the vertical resolution and lateral continuity of elastic parameters. This method has obtained good results in the prediction of carbonate reservoirs in the Lower Permian in the central Sichuan Basin. The lithological interface described is reasonable, especially the distribution characteristics of dolomite reservoirs. It enriches the research ideas for dolomite reservoirs prediction, and it has practical applicability to take advantage of multi-wave data in carbonate reservoir prediction.KeywordsPP+PS joint pre-inversionDolomite reservoirsConverted wavePP-PS wave correlation

In recent years, with the fine exploration of carbonate reservoirs in the Qixia Formation, central Sichuan Basin, researchers have made important oil and gas discoveries in the dolomite reservoirs. However, the characteristics, genesis and controlling factors of dolomite reservoirs in the Qixia Formation in this area are still unclear. In this paper, the petrology, pore structures, physical properties and geochemical characteristics of the dolomite reservoirs in the Permian Qixia Formation in the central Sichuan Basin have been systematically studied based on a large number of cores, thin sections, physical property and geochemical tests. Furthermore, the genesis and main controlling factors of dolomite reservoirs are clarified. The study shows that the dolomites have the characteristics of small single-layer thickness and multi-layer development, and they are usually interbeded with the limestones. The reservoir types are mainly fine crystal and fine medium crystal dolomite, and the reservoir spaces include intercrystalline pores and dissolution pores (or caves). Moreover, the reservoir physical properties are characterized by medium porosity and medium to high permeability. Quasi-syngenetic dolomitization is the main origin of dolomite in the target layer, and the dolomite was slightly modified by hydrothermal solution at the end of the Maokou Formation period. The development of dolomite reservoirs in the Qixia Formation in the study area is affected by palaeogeomorphology, sedimentation and diagenesis. The granular shoal facies developed in the high parts of the paleogeomorphology provides the material basis for the formation of dolomite reservoirs. The high frequency sequence interface controls the development of dolomite reservoirs in the highstand systems tract. In addition, the quasi-syngenetic dissolution promots the development of the early secondary pores and provids an effective channel for the migration of the diagenetic fluids in the later stage. In the direction of orthogonal horizontal deposition, the dolomites have the characteristics of thin monolayer thickness, multiple sequences and are interbedded with limestone. The dolomite shoals in the study area are distributed as clumps in the plane, which has great potential for hydrocarbon exploration.