An occurrence of coked bitumen, Raton Formation, Purgatoire River Valley, Colorado, U.S.A.
An occurrence of coked bitumen, Raton Formation, Purgatoire River Valley, Colorado, U.S.A.
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DOI：
10.1016/j.coal.2015.02.010
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发表时间：
2015-03
影响因子：
5.6
通讯作者：
S. M. Rimmer;J. Crelling;Lois E. Yoksoulian
中科院分区：
文献类型：
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作者：
S. M. Rimmer;J. Crelling;Lois E. Yoksoulian
据报道，科罗拉多州中南部的西班牙峰地区有许多因火成岩侵入煤炭而产生焦炭的例子。然而，在最近对科罗拉多州梅迪纳广场附近普尔加托瓦尔河沿岸拉顿组（上白垩纪-古新世）侵入部分的一项研究中，观察到焦化沥青。这种材料出现在煌斑岩基岩内页岩捕虏体的“手指”（六边形节理体）中以及基岩正下方的碳质 III 型页岩中。焦指的特点是具有显着的流镶嵌结构、高镜质体/焦炭反射率（平均随机反射率在 8% 到 9% 之间，但最大读数在 14% 到 15% 左右）、高各向异性、丰富的脱挥发分空泡以及缺乏惰性包裹体。在下面的页岩内，焦化沥青以孔隙、空隙和裂缝衬里和填充物的形式出现。从地球化学角度来看，手指中的焦炭具有较低的 S1 和 S2 值（分别 < 0.2 和 < 3 mg HC/g），以及较低的 HI 和 OI 值（分别 < 6 mg HC/g TOC 和 < 3 mg CO2/g TOC）。在手指内部，有证据表明存在多个阶段的积累，包括粗粒圆形或带状焦炭，通常含有气相沉积碳（热解碳）边缘的孔、热解碳层、球状热解碳簇和高度多孔的焦炭层。这种焦化沥青与同产地的焦煤有很大不同。焦煤具有中粒圆形镶嵌结构，与该地区未蚀变煤的高挥发性烟煤等级一致。煤源焦具有类似的脱挥发分空泡，但也含有大量惰性岩显微组分，如熔铁矿和分泌石。先前报道的流经同一区域的一个煤“堤”也显示出圆形马赛克纹理。这些观察结果表明，本研究中发现的焦炭不是由煤炭直接焦化形成的，而是由随后因侵入而焦化的流动相（沥青或沥青）形成的。这种沥青要么来自于岩床上方区域内丰富的煤炭，要么来自于 III 型碳质页岩中所含的有机质，或者两者兼而有之。在此地点采样的未改变的 III 型页岩的 HI 约为 350 mg HC/g TOC，表明具有一定的石油生成能力（尽管有限）。我们认为沥青是在侵入之前或侵入期间产生的，随后被就位的基坑的高温焦化。当沥青在基台附近聚集时，快速加热导致中间相的发展，导致冷却时形成高度各向异性的带状镶嵌结构，气体蒸发，随后沿着孔隙、空泡和裂缝凝结为热解碳和球状热解碳。下面的粉质页岩内的沥青也被焦化，但程度较轻。这种焦化沥青的质地看起来非常像商业生产的石油焦。这种焦化沥青与其他报道的焦化沥青的不同之处在于其存在方式（捕虏体中的焦指）、带状镶嵌结构以及极高的各向异性和反射率。
Numerous examples of coke produced by igneous intrusion into coal have been reported in the Spanish Peaks region of south central Colorado. However, in a recent study of an intruded section of the Raton Formation (Upper Cretaceous-Paleocene) along the Purgatoire River near Medina Plaza, CO, coked bitumen has been observed. This material occurs in “fingers” (hexagonally jointed bodies) in a shaley xenolith within a lamprophyre sill and in carbonaceous Type III shale directly below the sill. The coke fingers are characterized by a remarkable flow mosaic texture, high vitrinite/coke reflectance (average random reflectance between 8% and 9%, but with maximum readings around 14–15%), high anisotropy, abundant devolatilization vacuoles, and an absence of inertinite inclusions. Within the underlying shales, the coked bitumen occurs as pore, void, and fracture linings and fillings. Geochemically, the coke in the fingers has low S1and S2values (< 0.2 and < 3 mg HC/g, respectively), and low HI and OI values (< 6 mg HC/g TOC and < 3 mg CO2/g TOC, respectively). Within the fingers, there is evidence for multiple stages of accumulation, including coarse-grained circular or ribbon coke frequently containing pores edged by vapor-deposited carbon (pyrolytic carbon), layers of pyrolytic carbon, clusters of spherulitic pyrolytic carbon, and layers of highly porous coke. This coked bitumen is quite different from coked coal from the same locality. The coked coal has a medium-grained circular mosaic texture that is consistent with the high volatile bituminous rank of unaltered coal in the area. The coal-derived coke has similar devolatilization vacuoles but also has numerous inclusions of inertinite macerals such as fusinite and secretinite. A previously reported coal “dike” that had flowed through a sill in the same area also showed circular mosaic texture.These observations suggest that the coke found in this study was not formed by the direct coking of coal, but from a mobile phase (bitumen or pitch) that was subsequently coked by the intrusion. This bitumen was either derived from the abundant coal within the section above the sill, or from the organic matter contained within the Type III carbonaceous shales, or both. Unaltered Type III shale sampled at this site has a HI of ~ 350 mg HC/g TOC suggesting some, albeit limited, capacity to generate petroleum. We suggest that bitumen was generated prior to or during the time of intrusion and was subsequently coked by the high temperatures of the emplaced sill. As the bitumen pooled adjacent to the sill, rapid heating led to the development of mesophase, resulting in a highly anisotropic ribbon mosaic texture on cooling, and vaporization of gases that subsequently condensed along pores, vacuoles, and fractures as pyrolytic carbon and spherulitic pyrolytic carbon. Bitumen within the underlying silty shales was also coked, but to a lesser degree. The texture of this coked bitumen looks very much like commercially produced petroleum coke. This coked bitumen differs from other reports of coked bitumen in its mode of occurrence (coke fingers in a xenolith), ribbon mosaic structure, and extremely high anisotropy and reflectance.