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Effect of diabase intrusion on the CBM-bearing coalfields of southern Brazil: A review
Journal of South American Earth Sciences ( IF 1.8 ) Pub Date : 2022-09-22 , DOI: 10.1016/j.jsames.2022.104041
M.M. Bicca, W. Kalkreuth, A.L. Rosa, G. Simão, J. Levandowski, T.F. Silva, H.M. Anzolin

The Paraná Basin coalfields of southern Brazil belong to the Permian Gondwana I Supersequence interval. The coals generally experienced low burial temperatures at the basin edges, but local igneous intrusions of the Paraná superplume (Cretaceous) altered their previous thermal conditions. The Santa Terezinha, Chico-Lomã, and Morungava coalfields are considered to be the most proliferous thermally altered deposits for Coalbed Methane (CBM) prospection in the south of Brazil. To better understand the coalification process for these unconventional gas reservoirs, the present review presents a compilation of vitrinite reflectance (VR) measurements generated in earlier studies, coal petrological characteristics, and ash determination by proximate analysis of coal seams of the Rio Bonito Formation obtained from 5 well core profiles. The VR values were compared with the available desorption and adsorption volumes of gas, lithological and compositional data to understand the effect of intrusions on coal seams. The thermal halo dimension varies from 1 to 1.5 time the thickness of the igneous body. Both below and above the intrusion. The higher thermal halo was found in samples of the CBM001-CL-RS borehole. The analyses indicate that maceral and ash contents do not show significant change caused by the thermal halo, except for an increase in VRs near the igneous rock ranging from 1.0 to 5.0%Rr, reflecting thermal alteration by the intrusive bodies, forming natural coke near the contact. In samples not affected by the intrusion the VR range from 0.5 to 0.8%Rr, considered to reflect the values derived from burial history. Thermal calculations determined maximum paleotemperatures between 300 and 350 °C closer to the intrusion contact (2–5 m) and decreasing to average values within 100 ± 10 °C, with little or no effect of the intrusion heat. Unaffected samples of the Santa Terezinha Coalfield showed elevated paleotemperatures (120 ± 10 °C) compared to the other deposits, which implies higher subsidence rates or a local geothermal gradient increase. The content of CBM in coal seams generally increases with VRs and vitrinite volumes but decreases locally towards the contact with the intrusion. This behavior probably relates to the development of devolatilization vacuoles that reduce the adsorption surface of vitrinite macerals. There are seams with higher vitrinite contents that under proper thermal conditions, have more potential for gas generation and storage. The increase in depth and thickness of the coal seams also contribute to an increase in gas storage capacity.



中文翻译:

辉绿岩侵入对巴西南部含煤层气煤田的影响:综述

巴西南部的巴拉那盆地煤田属于二叠纪冈瓦纳I 超层序层段。煤在盆地边缘的埋藏温度普遍较低,但巴拉那超羽流(白垩纪)的局部火成岩侵入改变了它们以前的热条件。Santa Terezinha、Chico-Loma 和 Morungava 煤田被认为是巴西南部煤层气 (CBM) 勘探中最丰富的热蚀变矿床。为了更好地了解这些非常规气藏的煤化过程,本综述提出了镜质体反射率(VR) 早期研究中产生的测量值、煤岩学特征和灰分测定,通过对从 5 口井岩心剖面获得的 Rio Bonito 组煤层进行近似分析。将 VR 值与可用的气体解吸和吸附体积、岩性和成分数据进行比较,以了解侵入物对煤层的影响。热晕的尺寸是火成体厚度的 1 到 1.5 倍。在入侵的下方和上方。在 CBM001-CL-RS 钻孔的样品中发现了较高的热晕。分析表明, maceral和灰分含量没有表现出由热晕引起的显着变化,除了在火成岩附近的 VRs 增加 1.0 到 5.0%Rr,反映了侵入体的热改变,在接触附近形成了天然焦炭。在不受入侵影响的样本中,VR 的范围为 0.5 到 0.8%Rr,被认为反映了从埋藏历史得出的值。热计算确定了接近侵入接触点(2-5 m)的最大古温度在 300 到 350 °C 之间,并在 100 ± 10 °C 范围内降低到平均值,侵入热的影响很小或没有影响。与其他矿床相比,未受影响的 Santa Terezinha 煤田样品显示出较高的古温度(120 ± 10 °C),这意味着更高的沉降率或当地地温梯度增加。煤层中煤层气的含量通常随着 VRs 和镜质体体积的增加而增加,但随着与侵入体的接触而局部减少。这种行为可能与减少镜质体吸附表面的脱挥发分液泡的发展有关。有较高镜质体含量的煤层,在适当的热条件下,更有可能产生和储存气体。煤层深度和厚度的增加也有助于增加储气能力。

更新日期:2022-09-24
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