准噶尔盆地西北缘大侏罗沟走滑断层油气成藏效应

走滑断层判识及其油气成藏效应是当前构造地质学与石油地质学研究的热点。为进一步认识准噶尔盆地西北缘地区的控藏断层大侏罗沟走滑断层,对其进行了野外露头观察和地震剖面解释,以进一步明确其构造属性与边界,在此基础上,结合油层分布与原油性质及地球化学特征分析,讨论了其成藏效应。结果表明,大侏罗沟断层走滑特征明显,西端野外露头上断线平直、断面陡立、发育水平擦痕,东端地震剖面平面上发育"马尾状构造"、剖面上发育"花状构造"。据此重新限定了断层的边界,西以达尔布特断层为界,东以达探1井为界,全长约80km。大侏罗沟走滑断层对油气兼具横向和纵向输导效应,使得油气藏在平面上沿断层被动盘富集,纵向上呈"串珠状"分布,这可能是这类走滑断层所具有的普遍特征。据此,研究区一系列走滑断层的被动盘在下步勘探中值得重点关注。     

鄂尔多斯盆地奥陶系马家沟群块状白云岩的包体研究

鄂尔多斯盆地奥陶系马家沟群的块状白云岩，按晶粒结构可分为细硝糖状白云岩（晶粒大小为０．０１－０．１ｍｍ）和粗砂糖状白云岩（晶粒大小为０．０５－０．２５ｍｍ）。包体分析为块状白云岩的成因研究提供了最直接的信息，细砂糖状白云岩的主体白云石中液态包体不发育，但孔隙充填方解石中的包体发育，由２４个孔隙充填方解石中的包体测出均一温度为４９－７４℃，测算的包体形成深度的最大值为１７２ｍ。由此可以推断，其主体白     

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ¹⁸Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ¹³C values together with ⁸⁷Sr/⁸⁶Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ¹⁸Ο and δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ¹⁸Ο, δ¹³C and ⁸⁷Sr/⁸⁶Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.     

白云岩化研究的新进展

近年来,随着沉积学理论的发展,白云岩化的研究取得了重大进展,包括白云岩化的物质来源、白云岩化环境、白云岩化阶段和白云岩化机理等。其中以白云岩化机理中与区域构造运动有关的构造热液白云岩化、与火山活动有关的火山热液白云岩化和与变质作用有关的变质热液白云岩化最为重要。     

黔西南低温成矿域中不同层位不同类型金矿的内在统一成矿机制探讨

黔西南地区是扬子地台西南缘重要且极有远景的低温成矿域，以发育金矿为主。其低温是因为该区岩浆岩露头极少，相对远离直接的岩浆作用。矿体均赋存于地层中，具多时代多层位赋矿特征，成矿温度低于300℃，矿体受深大断裂控制明显，矿床类型以卡林型金矿为主，次有凝灰岩型金矿。但是，两类矿床的原生矿石性质一致，而且成矿时代是基本一致的，二者只是赋矿层位不同而已。据此研究表明，该区不同层位、不同类型金矿是受深大断裂和深源流体统一制约，并伴随深源流体改造地壳岩石而发生多期多源叠加成矿的产物。     

Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ¹⁸O values from ?13·7 to ?9·7‰ VPDB, δ¹³C values from ?2·7 to + 1·5‰ VPDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (T_h) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ¹⁸O values (?14·2 to ?9·3‰ VPDB), lower δ¹³C values (?3·0 to ?0·7‰ VPDB), higher ⁸⁷Sr/⁸⁶Sr ratios (≈ 0·7100) and higher T_h values (171–209 °C), which correspond to trapping temperatures (T_r) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ¹⁸O values (?18·5 to ?14·3‰ VPDB), variable δ¹³C values (?11·3 to ?1·2‰ VPDB) and high T_h values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).     

加拿大阿尔伯达省西部上泥盆统凯恩组白云岩的成因

加拿大阿尔伯达省西部上泥盆统凯恩组主要由粉晶白云岩和细晶白云岩组成，另外还常见一种充填溶蚀孔洞的亮晶白云石。粉晶白云岩纹理、泥裂和鸟眼等构造发育，其δ＾１３Ｃ为＋１．０‰－＋３．０‰，δ＾１８Ｏ为－６．０‰－－４．３‰，为潮坪上混合水白云化形成。细晶白云岩分布于假整合面之下，其δ＾１３Ｃ为＋０．２‰－＋２．９‰，δ＾１８Ｏ为－６．９‰－－４．６‰，也是混合水白云化的产物。亮晶白云石洁净明亮、晶粒粗     

地质记录中的海水白云岩化作用

地质记录中的海水白云岩化作用是广泛存在的。它不仅发育于时代较新的始新世地层内(如太平洋中的 Enewetak 珊瑚岛地区),而且也发育于时代较老的震旦纪灯影期地层内(如云南东北部地区)。在以前的白云岩成因研究过程中,海水白云岩化作用的普遍性和重要性被大大地低估了。自从 Saller(1984)首次报道了正常海水白云岩化的实例以来,海水白云岩化作用的研究已取得了长足的进展,如 Kasner(1984);Land(1985);Smart(1988);Tucker(1990);朱同兴、罗安屏(1993)等。近年来,作者通过对云南东北部地区震旦系灯影组潮缘沉积物的岩石学、沉积相、古地理格局和沉积地球化学标志等综合研究,认为该套巨厚的白云岩层(400—1000m)的成因不是原生的,而是成岩交代形成的,其主要的形成机制之一就是成岩早期的海水白云岩化作用。海水白云岩化作用的发育主要受海平面变化、沉积作用类型以及古气候等条件的限制。造成滇东北地区灯影期海水白云岩化作用的主要机制可能是位于其西北部的青藏大洋内的冷洋流向东部沉积物内泵吸,和位于其东南缘的扬子边缘海的海水向西部沉积物内循环对流(柯特对流)。上述的泵吸作用与对流作用的相互叠加可使巨大体积的海水通过扬子海域,尤其是扬子台地西南边缘(滇东北地区)沉积物,从而使其发生巨大规模的海水白云岩化作用。由海水白云岩化作用形成的白云岩的结构有序度中等,I=0.68—0.82,其碳、氧同位素组成均显示海水碳酸盐来源,δ~(13)=+1.8—+2.6‰(PDB),δ~(18)O=+2.1—+2.7‰(PDB)(冷洋流来源,Enewetak Atoll,始新统)或-5.1—-6.5‰(受地热和大气淡水双重影响而偏负值,滇东北地区;灯影组)。灯影期海水白云岩化作用模式合理地解释了区域巨厚沉积物的块状白云岩化成因、充足的镁离子来源以及多余的钙离子的搬运等白云岩成因研究过程中的关键性问题。     

Late Pliocene to Holocene platform evolution in northern Belize, and comparison with coeval deposits in southern Belize and the Bahamas

Lithostratigraphy, depositional facies architecture, and diagenesis of upper Pliocene to Holocene carbonates in northern Belize are evaluated based on a ca 290 m, continuous section of samples from a well drilled on Ambergris Caye that can be linked directly to outcrops of Pleistocene limestone, and of overlying Holocene sediments. Upper Pliocene outer‐ramp deposits are overlain unconformably by Pleistocene and Holocene reef‐rimmed platforms devoid of lowstand siliciclastics. Tectonism controlled the location of the oldest Pleistocene platform margin and coralgal barrier reef, and periodically affected deposition in the Holocene. A shallow, flat‐topped, mostly aggradational platform was maintained in the Holocene by alternating periods of highstand barrier‐reef growth and lowstand karstification, differential subsidence, and the low magnitude of accommodation space increases during highstands. Facies in Pleistocene rocks to the lee of the barrier reef include: (i) outer‐shelf coralgal sands with scattered coral patch reefs; (ii) a shoal–water transition zone comprising nearshore skeletal and oolitic sands amidst scattered islands and tidal flats; and (iii) micritic inner‐shelf deposits. Four glacio‐eustatically forced sequences are recognized in the Pleistocene section, and component subtidal cycles probably include forced cycles and autocycles. Excluding oolites, Holocene facies are similar to those in the Pleistocene and include mud‐mounds, foraminiferal sand shoals in the inner shelf, and within the interiors of Ambergris and surrounding cayes, mangrove swamps, shallow lagoons, and tidal and sea‐marginal flats. Meteoric diagenesis of Pliocene and Pleistocene rocks is indicated by variable degrees of mineralogic stabilization, generally depleted whole‐rock δ¹⁸O and δ¹³C values, and meniscus and whisker‐crystal cements. Differences in the mineralogy and geochemistry of the Pliocene and Pleistocene rocks are attributed to variable extent of meteoric alteration. Dolomitization in the Pliocene carbonates may have begun syndepositionally and continued into the marine shallow‐burial environment. Positive dolomite δ¹⁸O and δ¹³C values suggest precipitation from circulating, near‐normal marine fluids that probably were modified somewhat by methanogenesis. Sedimentologic and diagenetic attributes of the Pliocene–Pleistocene rocks in the study area are similar to those in the Bahamas with which they share a common history of sea‐level fluctuations and climate change.     

Genesis of island dolostones

Cenozoic ‘island dolostones’ are found on islands throughout the oceans of the world. Due to their geological youth and lack of deep burial, these dolostones provide an opportunity to resolve some of the mysteries surrounding the dolomite problem. In island dolostone bodies, which are of variable size and variable dolomitization, the petrographic and geochemical properties of the dolostones are characterized by geographic and stratigraphic variations. In the larger island‐wide dolostone bodies, like those found on Grand Cayman, there are progressive increases in mole %Ca (%Ca_mean: 53·9 to 57·6%), depletion of the heavier ¹⁸O and ¹³C isotopes (δ¹⁸O_mean: 3·6 to 2·1‰ _VPDB; δ¹³C_mean: 3·1 to 1·4‰ _VPDB), and changes from fabric‐retentive to fabric‐destructive fabrics and a decrease in the amount of dolomite cement from the coastal areas towards the centres of the islands, similar to the Little Bahama Bank. These changes define geographically concentric zones that parallel the coastlines and reflect geochemical modification of the dolomitizing fluid through water–rock interactions, mixing with meteoric water and the changes in the rate and flux of seawater as it flowed from coasts to island interiors. The pattern of dolomitization, however, is not consistent from island to island because geographic and stratigraphic variations, specific to each island, influenced groundwater flow pattern (for example, geometry and size of the islands; the porosity and permeability of the precursor limestone), the duration of the dolomitization reaction, and other factors. The geographic extent of dolomitization and variation in dolomite stoichiometry of island dolostones may be comparable to the reaction stages established in high‐temperature laboratory experiments.