North Carnarvon盆地三叠系沉积格局转换与烃源岩发育特征

North Carnarvon盆地是澳大利亚最主要的产油气盆地,也是世界上主要的富气盆地之一.三叠纪North Carnarvon盆地属于克拉通边缘坳陷,地层厚度巨大且平面分布广.中三叠世晚期澳大利亚西北陆架发生强制性海退,造成North Carnarvon盆地沉积格局发生转变,沉积环境由滨浅海突变为海陆过渡环境,从而沉积了两套差异性明显的地层,即Locker shale 和Mungaroo组.本文从宏观和微观、有机与无机入手,阐述了Locker shale 和Mungaroo组的古地理背景、沉积充填演化及其相关的烃源岩与有机质显微组分发育特征.宏观上,Mungaroo组发育大型三角洲,中粗粒分支流水道砂岩与暗色泥岩频繁互层,其中近端三角洲平原泥岩厚度较薄,薄煤层只局部发育,陆源有机质含量较高,平均TOC含量为1.59%;远端三角洲平原泥岩厚度较大,薄煤层广泛发育,陆源有机质含量高,TOC含量最高,达4.11%;三角洲前缘泥岩厚度较大,陆源有机质含量低,TOC含量低,为1.05%.Locker Shale以滨浅海相沉积为主,局部发育小型三角洲,其中开阔浅海亚相烃源岩厚度较大,前三角洲、滨浅海、开阔浅海三个亚相TOC含量相当,平均值为1.16%.微观上,Mungaroo组由近端平原亚相—远端平原亚相—前缘亚相壳质组含量逐渐增多,镜质组先增加后下降,惰质组先减少后增加;远端三角洲平原镜质组含量最高.Locker shale由前三角洲相—浅海相壳质组增加,镜质组和惰质组均减少.研究区三叠系烃源岩及有机质显微组分的分布规律与特提斯南缘特殊的古地理、季风洪水的古气候背景密切相关,Mungaroo组的远端三角洲平原为最有利的烃源岩发育环境. North Carnarvon 盆地三叠系岩相古地理格局与沉积充填演化规律体现了温室气候效应下阵发性水流主导的沉积特色,为类似地质背景下油气资源评价提供了新的思路.

The Transformation of Triassic Sedimentary Framework and Corresponding Source Rock,North Carnarvon Basin

The North Carnarvon basin is Australia's most important oil and gas basin, also one of main gas- rich basins in the world. In Triassic, North Carnarvon basin belongs to pericratonic depression and the strata are hugely thick and widely distributed. Mandatory regression occurred in northwest shelf of Australia in Late middle Triassic, causing the transformation of Triassic sedimentary framework, North Carnarvon basin, and the depositional environment of North Carnarvon basin from offshore turn into the transition facies, thus two distinct strata-Locker shale and Mungaroo group deposited in North Carnarvon basin. In this article, paleogeographic setting of Locker shale and Mungaroo group and characteristics of associated source rock and organic maceral are elaborated under macro-micro scales, and organic-inorganic aspects.On a macro level, Mungaroo group is dominated by large-scale delta interbedded distributary channel sandstones and dark mudstones. The mudstone in proximal delta plain is thin, of which terrigenous organic matter content is high and TOC content can be up to 1.59%, besides, thin coal seams just developed locally. The mudstone in remote delta plain is thick, of which terrigenous organic matter content is high and TOC content is highest and can be up to 4.11%, besides, thin coal seams developed widely. The mudstone in delta front is thick, of which terrigenous organic matter content is low and TOC content can be up to 1.05%. Locker Shale is composed of major shallow sea depositional association and small-scale delta. The source rock in open shallow sea subfacies is the thick, what's more, prodelta, offshore and open shallow sea subfacies TOC content is much lower, and three sub-phases average TOC is 1.16%.On a micro level, sedimentary facies of Mungaroo group from the proximal plain facies-distal plain facies-front subfacies liptinite content gradually increased, with that vitrinite first increased and then decreased, but inertinite first decreased and then increased. As Locker Shale, by the prodelta to neritic facies liptinite increased, with that both vitrinite and inertinite reduced.The distribution of source rocks and organic maceral is closely related to special paleogeographic and monsoon flood paleoclimate setting of the southern margin of Thethys. The distal delta plain of Mungaroo group is the most favorable environment for the development of source rock. Lithofacies palaeogeographic framework and Sedimentary evolution regularity in Trassic of North Carnarvon Basin reflect the sedimentary characteristics which is dominated by paroxysmal water under the effect of greenhouse climate, providing a new train of thought for oil and gas resource evaluation in a similar geological setting.