Characteristics of shale deposition in relation to stratigraphic sequence systems tracts
Characteristics of shale deposition in relation to stratigraphic sequence systems tracts
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作者：
S. Schutter
地层层序的各种体系域包含具有规律性变化和可预测特征的不同页岩。它们的沉积组构、古生物特征和矿物学特征与它们的层序地层位置有关，因此可以用作经济评价以及评价水文地质和工程性质的建模工具。在露头上观察到的特征可用于解释层序内的背景，而对整个层序的了解可用于预测局部页岩特征。与低位扇有关的页岩包括浊积岩之间的半远洋沉积物以及由减弱的浊积岩流沉积的页岩。半远洋页岩在矿物学和有机质含量方面是原地的，而浊积岩中的页岩是异地的，并且可能具有对比的矿物学和有机质类型。低位扇页岩可能是重要的垂向渗透率屏障。古土壤与低位沉积有关。它们可能是孔隙带或渗透屏障，但通常是工程特性的薄弱区。红土风化作用可能产生有经济价值的矿床。低位楔状页岩不同于高位页岩，因为它们在沉积后不暴露。沉积仅在准层序规模上是横向的，低位楔通常向上变得更富含页岩。高页岩含量可能使它们成为重要的盖层或弱透水层。海侵体系域往往是改造的，泥质贫乏，有时只有砂层或壳层滞后的沉积，但在高沉积区可能沉积厚泥质楔形体。这种沉积可能有利于不寻常的生物群化（如伊利诺伊州的宾夕法尼亚州马松溪生物群或不列颠哥伦比亚省的寒武纪伯吉斯页岩动物群），因为快速埋藏而没有随后的淡水流入。这种页岩楔还可以保护煤不受随后与富含硫酸盐的海水的相互作用。凝析岩段的页岩是最有特色的。它们通常沉积在海侵晚期和高水位早期的体系域中，但具有足够不同的特征，需要单独考虑。通常，它们含有很少或没有粗陆源碎屑，但富含化石碎屑（特别是磷酸盐物质），有机物和化学沉淀物。因此，它们可能是烃源岩或含有层控矿。它们可能完全被生物扰动，或者如果在厌氧条件下沉积，它们可能极易裂变。连续性非常好，它们可能是重要的油气盖层或弱透水层。高水位体系域中沉积的页岩通常为前三角洲至冲积，并逐渐变为桑迪和非均质。在碳酸盐沉积占主导地位的地区，页岩可能变成泥灰岩并逐渐变成石灰岩。风暴可能会产生交替的页岩和砂岩或石灰石薄层。低连续性和非均质岩性使经济价值最小化。
The various systems tracts of stratigraphic sequences contain different shales with regularly variable and predictable characteristics. Their sedimentary fabrics, paleontologic features and mineralogy are related to their sequence stratigraphic position and so can be used as modeling tools for economic assessment as well as evaluating hydrogeologic and engineering properties. Features observed on outcrop can be used to interpret the setting within the sequence, while a knowledge of the overall sequence can be used to predict local shale characteristics. Shales associated with lowstand fans include the hemipelagic deposits between turbidites as well as the shales deposited by waning turbidite flows. The hemipelagic shales are autochthonous in terms of mineralogy and organic content, while the shales in the turbidites are allochthonous and may have contrasting mineralogy and organic material type. Lowstand fan shales may be important vertical permeability barriers. Paleosols are associated with lowstand deposition. They may be either porosity zones or permeability barriers, but are usually zones of weakness in engineering characteristics. Economically valuable deposits may be produced by lateritic weathering. Lowstand wedge shales differ from highstand shales because they are not exposed after deposition. Deposition is only progradational on a parasequence scale, and lowstand wedges generally become more shale-rich upward. The high shale content may make them important seals or aquitards. Transgressive systems tracts are often reworked and shale-poor, sometimes with only palimpsest deposits of sand or a shelly lag, but in areas of high sedimentation thick shaly wedges may be deposited. This deposition may favor unusual fossilization (such as the Pennsylvanian Mazon Creek biota of Illinois or the Cambrian Burgess Shale fauna of British Columbia) due to rapid burial without a subsequent freshwater influx. Such shale wedges may also protect coals from subsequent interaction with sulfate-rich seawater. Shales in condensed sections are the most distinctive. They are usually deposited during the late transgressive and early highstand systems tracts, but have sufficiently different characteristics to be considered separately. Typically, they contain little or no coarse terrigenous clastics, but are enriched in fossil debris (particularly phosphatic material), organics and chemical preciptates. Thus, they may be hydrocarbon source rocks or contain stratabound ores. They may be completely bioturbated or they may be extremely fissile if deposited under anaerobic conditions. Continuity is very great and they may be important hydrocarbon seals or aquitards. Shales deposited in highstand systems tracts are usually prodeltaic to alluvial and increasingly sandy and heterogeneous. In areas where carbonate deposition is dominant, the shales may become marly and grade into limestones. Storms may produce alternating thin beds of shale and sandstone or limestone. The low continuity and heterogeneous lithology minimize economic value.