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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >PASSIVE MARGIN EVOLUTION AND CONTROLS ON NATURAL GAS LEAKAGE IN THE ORANGE BASIN, SOUTH AFRICA
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PASSIVE MARGIN EVOLUTION AND CONTROLS ON NATURAL GAS LEAKAGE IN THE ORANGE BASIN, SOUTH AFRICA

机译：南非橙色盆地的被动边际演化及天然气泄漏控制

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摘要

Throughout exploration Block 2 of the Orange Basin offshore the South African continental margin, different natural gas leakage features and the relationship between natural gas leakage with structural and stratigraphic elements were studied. This study also quantifies liquid/gas hydrocarbon generation, migration and seepage dynamics through the post-rift history of the basin. The interpretation of seismic data reveals two mega-sequences: Cretaceous and Cenozoic that are subdivided by major stratigraphic unconformities into 5 and 2 sub-units, respectively. The basin is also divided into 2 structural domains: 1. an extensional domain characterized by basinward dipping listric normal faults rooted at the Cenomanian/Turonian level identified between 500 to 1500 m of present-day depth, 2. a compressional domain that accommodates the up-dip extension on the lower slope, and which is characterized by landward dipping thrust faults. One hundred and thirteen observed gas chimneys are identified and classified into stratigraphically-controlled (sa-c) and structurally-controlled (s-c) chimneys. The ratio of s-c versus s-ac chimneys is estimated as 2:5, which suggest a strong stratigraphic control on natural gas leakage. The chimneys either terminate at the seafloor where active leaking gas is manifested by pockmarks,or are sealed within the Miocene (14 Ma) sequence as paleo-pockmarks. The s-c chimneys are located along the normal faults in the extensional domain, and terminate as seafloor mounds up to 1500 m in diameter and with heights between 10 to 50 m. The sa-c pockmarks range between 100 to 400 m in diameter, and are linked to stratigraphic onlaps and pinch-outs within the Aptian sequence. Several giant chimneys, with diameters of more than 7 km, are also identified. At least one of these displays apparent internal gravitational collapse structures. Bright spots indicative of gas presence within these large chimneys were identified, but there is no evidence of acoustic turbidity or seismic pull-downs within these large structures. This suggests the giant chimneys are inactive paleo-gas-escape structures. Modelling suggests that gas from the lower Aptian and the Barremian source rocks migrates laterally-updip to the proximal parts of the basin where it accumulates beneath the Cenomanian/Turonian sequence that acts as a regional seal. Across the shelf-break and the upper slope, chimneys and pockmarks are fed from younger Cenomanian/Turonian source rocks. The migration model also indicates that fluids are about 24 times more likely to flow out of the study area than to be preserved within it. Since methane gas escaping across the sea floor into the exosphere (combined hydrosphere and atmosphere) may contribute to Earth’s climate fluctuations, and because escaping gas must have been cut off when at least half of identified s-c chimneys were sealed within the Miocene sequence, decrease of gas escape along the southern African continental margin may have to be factored into global Neogene cooling models.
机译：在整个南非大陆边缘奥兰治盆地第二块勘探的整个过程中，研究了不同的天然气泄漏特征以及天然气泄漏与结构和地层元素之间的关系。这项研究还通过盆地裂谷后的历史定量了液/气烃的产生，运移和渗流动力学。地震数据的解释揭示了两个大的层序：白垩纪和新生代，它们被主要的地层不整合分为五个子单元和两个子单元。该盆地也分为2个结构域：1.延伸域，其特征是在今天深度的500至1500 m之间确定的，以切诺曼尼/土伦阶为基础的盆地浸润性正断层，其特征为2。 -在下斜坡上的倾角延伸，其特征是向内倾覆的逆冲断层。确定了113个观察到的烟囱，并将其分为地层控制的（sa-c）和结构控制的（s-c）烟囱。 s-c烟囱与s-ac烟囱的比率估计为2：5，这表明对天然气泄漏进行了强有力的地层控制。烟囱要么终止于海底，在该海底处有痘痕，要么是活动性泄漏气体，或者是在中新世（14 Ma）层序中被密封为古痘痕。 s-c烟囱沿伸展区域的正常断层分布，并终止于直径达1500 m，高度在10至50 m之间的海底丘。 sa-c麻点的直径范围在100到400 m之间，并与Aptian层序中的地层重叠和收缩相关。还确定了几个直径超过7公里的巨型烟囱。这些中的至少一个显示出明显的内部重力塌陷结构。确定了指示这些大型烟囱中存在气体的亮点，但没有证据表明这些大型结构中存在声音混浊或地震波降。这表明巨型烟囱是不活跃的古气逃逸结构。模拟表明，来自下阿普提安和巴雷米亚烃源岩的天然气横向上倾向盆地的近端迁移，并在该盆地的塞诺曼尼亚/突尼斯序列之下聚积，作为区域封存。在架子断裂和上坡上，烟囱和麻点是由较年轻的西诺曼尼亚/突尼斯的烃源岩提供的。迁移模型还表明，流体流出研究区域的可能性比保存在其中的可能性高约24倍。由于甲烷气体通过海底逸出到外层大气（水圈和大气混合）可能会导致地球的气候波动，而且由于至少有一半的已识别sc烟囱被密封在中新世序列内时，必须将逸出气体切断，因此降低了南部非洲大陆边缘的天然气逸出可能必须纳入全球新近纪冷却模型中。

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