Thick Upper Cambrian-Lower Ordovician carbonates were deposited on a shallow marine platform in the northern Tarim Basin, which were extensively dolomitized, particularly for the Upper Cambrian ...carbonates. The resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites. Based on petrographic textures, matrix dolomites consist of very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), finely to medium crystalline, planar-e(s) dolomite (Md2), and finely to coarsely crystalline, nonplanar-a dolomite (Md3). Minor cement dolomites include finely to medium crystalline, planar-s(e) dolomite (Cd1) and coarsely crystalline, nonplanar saddle dolomite (Cd2), which partially or completely fill dissolution vugs and fractures; these cements postdate matrix dolomites but predate later quartz and calcite infills. Origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography, isotopic geochemistry (O, C and Sr), and fluid inclusion microthermometry. Md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines (e.g., evaporated seawater) in near-surface to very shallow burial settings, whereas Md2 dolomite was formed from connate seawater in association with burial dissolution and localized Mg concentration (or cannibalization) in shallow burial conditions. Md3 dolomite, however, was likely the result of intense recrystallization (or neomorphism) upon previously-formed dolomites (e.g., Md1 or Md2 dolomite) as the host carbonates were deeply buried, and influenced by later hydrothermal fluids. Subsequent cement dolomite and quartz crystals precipitated from higher-temperature, hydrothermal fluids, which were contributed more or less by the extensive Permian large igneous province (LIP) activity in Tarim Basin as evidenced by less radiogenic Sr in the cement and parts of matrix dolomites. This extensive abnormal hydrothermal activity could also have resulted in recrystallization (or neomorphism) on the previous matrix dolomites. Faults/fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths. However, the basin uplift triggered by the Late Hercynian Orogeny from the Late Permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids, resulting in precipitation of later calcites in which lighter C and more radiogenic Sr components demonstrate such a switch of fluid properties. This study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the Permian LIP activity within Tarim Basin and elsewhere.
•Three types of matrix dolomites and two types of cement dolomites, with minor other cement phases, were identified.•Matrix dolomites were formed in slightly evaporated seawater and connate seawater stored in formations.•Cement dolomites precipitated from hydrothermal fluids intimately related to the extensive Permian large igneous province (LIP) in Tarim Basin.•Recrystallization of matrix dolomites were likely associated with hydrothermal activity.
Massive dolostones pervasively occur in the Lower Paleozoic strata in Tarim Basin, NW China. This study focuses on the hydrothermally-altered dolostones in the Lower Ordovician at Sancha, Bachu ...County where the hydrothermally-altered dolostones, marbles and diabase intrusions co-occur within a large-scale, NW trending strike-slip fault zone. Within this zone, the intrusive diabases mostly occur along the master border faults to the east and west, respectively. The limestones that got in contact with the intrusions commonly were marbleized and the antecedent dolostones were subject to intense hydrothermal alteration, particularly along fractured portions. Based on field investigations and petrographic examinations on the hydrothermally-altered dolostones, five distinctive fabrics of matrix and cement dolomites are distinguished. Two episodes of vein-filling calcites postdate the hydrothermally-precipitated saddle dolomites. Integrated isotopic geochemistry (C, O and Sr) and fluid inclusion microthermometry suggest that dolomite recrystallization and precipitation were associated with the hydrothermal activity induced by fracturing/faulting and intrusive magmatism. CAMECA SIMS U–Pb isotopic dating of zircon grains from the diabases yields an age of 290.5 ± 2.9 Ma in the Early Permian, which reconciles the relative timing revealed by the paragenetic history. The magmatic emplacement was thus linked to the large igneous province (LIP) of Early Permian in Tarim Basin. In this case, the widespread magmatic activities could have caused the thermal anomaly and fracturing in the strata that were penetrated by the intrusions, enhancing the thermal diffusion and hydrothermal fluid migration (although minor volumes of thermal fluids from the basic magmas) through the fracture/fault systems, thereby resulting in contact metamorphism (or marbleization), recrystallization of matrix dolomites and dolomite precipitation commonly in primary dolostone successions. This study provides a useful analogue to understand the dolomitization and relevant issues for the deeply-buried dolostones associated with tectono-magmatism within the Tarim Basin and elsewhere.
•Hydrothermally-altered dolomites in the Lower Ordovician, Tarim Basin.•Integrated petrography, isotope geochemistry and fluid inclusion analyses conducted.•Hydrothermal alteration preferred in tensional fracture field.•Hydrothermal process linked to the tectono-magmatic event in the Early Permian.
Dolomites occur extensively in Cambrian to Lower Ordovician carbonates in the Tienshan orogen of the Quruqtagh area, north‐east Tarim Basin, where thick (up to 1 km), dark grey lenticular limestones ...of semi‐pelagic to pelagic origin are prominent. The dolomites generally occur as beige, anastomosed geobodies that cross‐cut well‐stratified limestones. Based on detailed field investigations and petrographic examination, three types of matrix dolomite are identified: fine crystalline planar‐e (Md1), fine to medium crystalline planar‐s (e) (Md2) and fine to coarse crystalline non‐planar‐a (Md3) dolomites. One type of cement dolomite, the non‐planar saddle dolomite (Cd), is also common. The preferential occurrence of Md1 along low‐amplitude stylolites points to a causal link to pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during shallow burial compaction. Type Md2, Md3 and Cd dolomites, commonly co‐occurring within the fractured zones, have large overlaps in isotopic composition with that of host limestone, implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water–rock interaction. However, the lower δ18O and higher 87Sr/86Sr ratios of these dolomites also suggest more intense fluid–rock interaction at elevated temperature and inputs of Mg and radiogenic Sr from the host limestones with more argillaceous matter and possibly underlying Neoproterozoic siliciclastic strata. Secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher‐temperature Mg‐rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting, forming irregular networks of dolomitized bodies enclosed within the host limestones. This scenario probably took place during the Late Hercynian orogeny, as the Tarim block collided with Tienshan island arc system to the north and north‐east. Subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the South Tienshan Ocean (or Palaeo‐Asian Ocean) and significant tectonic uplift of the Tienshan orogen. This study demonstrates the constructive role of notably tensional (or transtensional) faulting/fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin; however, a relatively short‐lived, low fluid flux may have limited the dolomitization exclusively within the fractured/faulted limestones in the overthrust sheets.
The Upper Ediacaran microbial carbonates of the Tarim Basin are potential reservoir geobodies for future hydrocarbon exploration with rising interest in exploration for deeply‐buried reserves. ...However, little knowledge has been acquired on the types of microbial carbonates that are present, the properties of the reservoir and the pore evolution, hampering predictions of high‐quality reservoirs in these carbonates. Integrated with petrography and in‐situ U‐Pb dating geochronology analyses, this study aims to clarify the types of microbial carbonates present and to reconstruct the pore evolution processes of the potential reservoir rocks. The Upper Ediacaran microbial carbonates of the Tarim Basin can be divided into four types, based on their features in terms of different scales (macro‐ to micro‐): microbial laminite, stromatolite, spongiomicrobialite and microbial‐peloidal wackestone/mudstone. Petrophysical properties show that all these microbial carbonates have low porosity and very low permeability with poor connectivity. These carbonates were subject to long‐term and complex diagenetic processes, mainly consisting of dissolution, compaction, pervasive dolomitization, cementation and fracturing. The most important reservoir spaces are contributed by vugs and dissolution‐enlarged pores, which are likely to have been associated with the widespread uplift of the Aksu area in the terminal Ediacaran. In contrast, the cementation of the fine‐to‐medium crystalline dolomite greatly reduced the pre‐existing pores. Pore types are closely related to different microbial fabrics, which played an important role in the pore evolution of the microbial carbonates.
High-yield natural gas was discovered in well SN4 in the Ordovician Yingshan Formation in the Tarim Basin. The gas is found in unusual, silicified, carbonate reservoirs. According to the degree of ...silicification, the silicified reservoirs can be divided into a lower section of silicified carbonates, a middle section of limestone, and an upper section of silicified carbonates. The silicified carbonates are mainly composed of quartz and calcite, in which the reservoir space mostly occurs as vugs, inter-crystalline pores of quartz, and partial fractures. Porosity varies widely, ranging from 3 to 20.5% with strong heterogeneity. The homogenization temperatures of fluid inclusions in quartz and calcite show that the silicification temperatures were 150–190°C, with characteristics of high temperature/low salinity and low temperature/high salinity. The 87Sr/86Sr ratios of secondary calcite are 0.709336–0.709732, which are significantly higher than that of concurrent seawater, indicating that the hydrothermal fluid originated from the deep clastic strata or the basement (sialic rock). The δ
13C values of the secondary calcite are similar to that of the surrounding limestone, indicating that the carbon in the secondary calcite is derived from the limestone strata, and that the secondary calcite is the product of dissolution and re-precipitation resulting from interaction between the silica-bearing hydrothermal fluids and surrounding limestones. The silicification of silica-bearing hydrothermal fluid was significantly controlled by strike-slip faults. The fluids ascending along the fault zone and branch faults interacted with the surrounding limestone in the Yingshan Formation. As a result, a large amount of quartz and secondary calcite were formed together with various types of secondary pores, resulting in excellent reservoirs.
► 3/15 Tazhong Uplift oils correlate with Cambrian–Lower Ordovician source rocks. ► 12/15 Tazhong and 39 Tabei oils correlate with Middle–Upper Ordovician source rocks. ► Free, adsorbed and inclusion ...oils have been recovered and analyzed by GC and GC–MS.
Molecular data from a large set of source rock, crude oil and oil-containing reservoir rock samples from the Tarim Basin demonstrate multiple sources for the marine oils in the studied areas of this basin. Based on gammacerane/C
31 hopane and C
28/(C
27
+
C
28
+
C
29) sterane ratios, three of the fifteen crude oils from the Tazhong Uplift correlate with Cambrian–Lower Ordovician source rocks, while the other crude oils from the Tazhong Uplift and all 39 crude oils from the Tahe oilfield in the Tabei Uplift correlate with Middle–Upper Ordovician source rocks. These two ratios further demonstrate that most of the free oils and nearly all of the adsorbed and inclusion oils in oil-containing reservoir rocks from the Tazhong Uplift correlate with Cambrian–Lower Ordovician source rocks, while the free and inclusion oils in oil-containing carbonates from the Tahe oilfield correlate mainly with Middle–Upper Ordovician source rocks. This result suggests that crude oils in the Tazhong Uplift are partly derived from the Cambrian–Lower Ordovician source rocks while those in the Ordovician carbonate reservoirs of Tahe oilfield are overwhelmingly derived from the Middle–Upper Ordovician source rocks.
The scatter of C
23 tricyclic terpane/(C
23 tricyclic terpane
+
C
30 17α,21β(H)-hopane) and C
21/(C
21
+
ΣC
29) sterane ratios for the free and inclusion oils from oil-containing carbonates in the Tahe oilfield possibly reflects the subtle organofacies variations in the source rocks, implying that the Ordovician reservoirs in this oilfield are near the major source kitchen. In contrast, the close and positive relationship between these two ratios for oil components in the oil-containing reservoir rocks from the Tazhong Uplift implies that they are far from the major source kitchen.
► GC–IRMS was performed for crude oils and components in source and reservoir rocks. ► The δ13C values show the marine oils are derived from two types of source rocks. ► The δ13C values for oil ...components in reservoir rocks reveal source variations. ► δ13C values do not correlate with biomarker parameters for mature oils.
Carbon isotopic compositions were determined by GC–IRMS for individual n-alkanes in crude oils and the free, adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the Tazhong Uplift and Tahe oilfield in the Tabei Uplift of Tarim Basin as well as extracts of the Cambrian–Ordovician source rocks in the basin. The variations of the δ13C values of individual n-alkanes among the 15 oils from the Tazhong Uplift and among the 15 oils from the Triassic and Carboniferous sandstone reservoirs and the 21 oils from the Ordovician carbonate reservoirs in the Tahe oilfield demonstrate that these marine oils are derived from two end member source rocks. The major proportion of these marine oils is derived from the type A source rocks with low δ13C values while a minor proportion is derived from the type B source rocks with high δ13C values. Type A source rocks are within either the Cambrian–Lower Ordovician or the Middle–Upper Ordovician strata (not drilled so far) while type B source rocks are within the Cambrian–Lower Ordovician strata, as found in boreholes TD2 and Fang 1. In addition, the three oils from the Cretaceous sandstone reservoirs in the Tahe oilfield with exceptionally high Pr/Ph ratio and δ13C values of individual n-alkanes are derived, or mainly derived, from the Triassic–Jurassic terrigenous source rocks located in Quka Depression.
The difference of the δ13C values of individual n-alkanes among the free, adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process. In general, the initial oil charge is derived from the type B source rocks with high δ13C values while the later oil charge is derived from the type A source rocks with low δ13C values.
The δ13C values of individual n-alkanes do not simply correlate with the biomarker parameters for the marine oils in the Tazhong Uplift and Tahe oilfield, suggesting that molecular parameters alone are not adequate for reliable oil-source correlation for high maturity oils with complex mixing.
The burial dissolution of carbonate rocks has long been an interesting topic of reservoir geologists. Integrated with geological studies and reactive transport modeling, this study investigated the ...Cambrian dolomites that were buried at depths up to 8408 m and still preserved a large amount of unfilled dissolution vugs from the borehole TS1 in the northern Tarim Basin. Studies indicate that these vugs were formed in association with fault-channeled hydrothermal fluids from greater depth through “retrograde dissolution” as the fluid temperature dropped during upward migration. The reactive transport modeling results suggest an important control of the vertical permeability of wall-rock on fluid and temperature patterns which, in turn, would control the spatial distribution of dissolving-originated porosity. The hydrothermal dissolution mainly occurred in dolomite wall-rocks with higher vertical permeability (extensive development of tensional fractures and connected pore spaces), producing additional dissolved porosity there during deep burial. This study implicates the importance of multidisciplinary approaches for understanding the burial/hydrothermal dissolution of dolomite rocks and predicting favourable deep/ultradeep carbonate reservoirs.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The high-angle, structural genetic calcite veins of the Ordovician in the Shunnan-Gucheng area in the Tarim Basin provide a convenient study of fluid properties, hydrocarbon migration, and ...accumulation events in the fault zone of this area. Based on detailed core investigation, studies on fluid inclusions in calcite veins and comparison of geochemical characteristics of limestone matrix and calcite veins were carried out. Multiple types of hydrocarbons inclusion (e.g., dry asphalt inclusion,three-phase hydrocarbon inclusion, oil and water inclusion,gas-rich inclusion) were found in calcite veins. Furthermore, the coexisted aqueous inclusions with the gas–liquid phase had a high homogenization temperature (130–160 °C). The distribution characteristics of 18O-depleted (δ18O: −13.1‰ to −8.7‰), 87Sr-enriched (87Sr/86Sr: 0.708 879 to 0.710 432), relatively high total Rare Earth Elements (REEs), and positive Europium Anomalies indicated that the structural origin of calcite veins was mainly derived from the dissolution of surrounding rocks and diagenetic-hydrocarbon fluids. Participation signs of meteoric water were not proved. The rich hydrocarbon inclusions in the calcite veins indicated that the north-east strike-slip fault zone in the Shunnan-Gucheng area is an essential channel for hydrocarbon migration and accumulation.
The saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in Upper Cambrian carbonate of western Tarim basin, suggesting that formation of the ...saddle dolomites is likely related to fault-controlled fluid flow. They partially fill in fractures and vugs of replacement dolomite. The saddle dolomites exhibit complex internal textures, commonly consisting of core and cortex. In comparison with the matrix dolomites, the saddle dolomites show lower Sr-content and
87Sr/
86Sr ratios, higher Fe- and Mn-content, and more negative
δ
18O values. Combined with high Th (100–130 °C) of primary fluid inclusions, it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite-bearing Middle Cambrian strata, and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration. Fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites. The cores precipitated from the formation water mixed by deep brines at the early stage of fault activity, and the cortexes precipitated from the deep fluid with higher temperatures through the Middle Cambrian later. In summary, the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity, which also resulted in high porosity in Upper Cambrian carbonate in western Tarim Basin.
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