Physical properties of petroleum formed during maturation of Lower Cambrian shale in the upper Yangtze Platform, South China, as inferred from PhaseKinetics modelling

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• 作者：Jingqiang Tan ; Brian Horsfield ; Nicolaj Mahlstedt ; Jinchuan Zhang ; Rol ; o di Primio ; Thi Anh Tiem Vu ; Christopher J. Boreham ; Ger van Graas ; Bruce Alastair Tocher
• 关键词：Lower Cambrian shale ; South China ; Shale gas ; Petroleum generation ; PhaseKinetics ; Phase behaviour
• 刊名：Marine and Petroleum Geology
• 出版年：December, 2013
• 年：2013
• 卷：48
• 期：Complete
• 页码：47-56
• 全文大小：2580 K

文摘

Lower Cambrian shale in the Upper Yangtze Platform (UYP), South China, is an important source rock of many conventional petroleum fields and was recently recognized as a promising unconventional shale reservoir. In this paper, hydrocarbon generation kinetics and petroleum physical properties were investigated using the PhaseKinetics approach () and a Cambrian shale sample from the Georgina Basin, North Territory Australia (NTA), as similar paleogeological and sedimentary environments in Cambrian are found for the UYP and NTA.

The source rock comprises type II kerogen and belongs to an organofacies generating Paraffinic-Naphthenic-Aromatic low wax oil. Bulk petroleum generation can be described by a single frequency factor A聽=聽8.43E聽+聽14 (1/s) and a dominant activation energy at 56聽kcal/mol, which is characteristic for sulphur-poor organic matter deposited in an anoxic marine environment. Onset (transformation ratio TR聽=聽10%) and end (TR聽=聽90%) of bulk hydrocarbon generation was calculated to take place at 120聽掳C and 165聽掳C respectively for an assumed average geological heating rate of 1.5掳C/Ma. Based on the thermal history of a local 鈥渕odel鈥?well, onset temperature was not reached until the Middle Triassic (241聽ma) when sediments were buried more than 2000聽m and basalt eruptions caused enhanced heat flows. The main generation stage of primary petroleum took place during the Middle-Late Triassic and ended in the Early Jurassic (187聽ma) for burial depths exceeding 4000聽m (TR 90%; 165聽掳C). Temperatures increased to more than 200聽掳C in the Middle-Late Jurassic leading to secondary cracking of primary products.

Hydrocarbons formed at the onset (TR聽=聽10%) of petroleum generation can be characterized by a gas-oil-ratio (GOR) of 63 Sm³/Sm³, a saturation pressure (P_sat) of 101聽bar, and a formation volume factor (B_o) of 1.2聽m³/Sm³. Those parameters stay low during primary petroleum generation before 203 ma, at temperatures < 145聽掳C, and at burial depths <3400聽m (GOR聽=聽176聽Sm³/Sm³, P_sat聽=聽189聽bar, B_o聽=聽1.6聽m³/Sm³ at 90% TR). However, predicted parameters increase rapidly (GORs聽>>聽10,000聽Sm³/Sm³, P_sat聽>聽250聽bar and B_o聽>聽2.0聽m³/Sm³) during secondary cracking starting roughly at 200 ma, 152聽掳C and 3500聽m burial. Assuming zero expulsion, the shale reservoir position within the sedimentary basin indicates that bubble point pressure was always below reservoir pressure, and fluids in the shale reservoir occurred only as a single, undersaturated phase throughout maturation history. Black oil and volatile oil phases dominated during the primary cracking period, whereas wet gas and dry gas phases dominated during the secondary cracking period.