川南地区龙马溪组页岩气富集高产主控因素分析

Analysis of main controlling factors for the enrichment and high productivity of the Longmaxi shale gas in Southern Sichuan Basin

川南地区龙马溪组是海相页岩气开发的重点,不同页岩气井产量差异较大。为揭示影响页岩气产量的主要地质因素,通过多因素分析,对川南地区页岩气富集高产的“沉积控气”、“超压控产”主控因素进行了系统研究。①沉积条件控制优质页岩的发育规模及产气能力,优质页岩连续厚度大于30 m,有机质丰度(TOC)高(>3.0%)、有机质类型好、热演化程度(R_(o))适中(1.8%<R_(o)<3.0%),是保证充足气源供给的前提条件;②沉积条件控制页岩的储集能力,孔缝发育且岩石脆性高可以提供良好储集条件;③沉积条件控制页岩气的保存条件,封闭性良好的顶底板或隔层是保证页岩气有效保存的重要条件。除此之外,地层超压是高含气量的表征,是实现页岩气高产的重要条件。因此,研究认为川南地区龙马溪组页岩气富集高产区位于构造稳定区的深水陆棚相沉积建造中。

The Longmaxi shale in the southern Sichuan Basin is the focus of marine shale gas exploration in China.Its features are“thick shale,stable distribution,rich in organic matter,ultra-high strata pressure,and good preservation conditions.”It has favorable conditions for the augmentation and large scale production of shale gas.The production level of different shale gas wells varies significantly.To further achieve large scale and highly efficient exploration and development of the Longmaxi shale gas production in this basin,the cause of the different yields should be examined.This depends on the analysis of the sedimentary distribution,thickness,organic geochemistry,reservoir characteristics,and preservation conditions,with a focus on the existing data of shale gas exploration and development.In this study,the main controlling factors for the enrichment and production of shale gas were systematically dissected through typical shale gas well data anatomy analysis.This can be summarized as“deposition to control gas”and“overpressure to control production.”First,depositional conditions control the development scale and quality of shale gas production.Factors,such as the depth of high-quality shale greater than 30 m,high organic matter content(TOC>3.0%),appropriate organic matter type,and moderate thermal evolution degree(1.8%<R_(o)<3.0%),are prerequisites for adequate gas supply in the development area.Second,sedimentary rock conditions control shale reservoir capacity.Well-developed pores and fractures and high rock brittleness provide good reservoir conditions.Good reservoir condition is the key factor to achieving large scale production of shale gas,as a high brittle mineral content enhances the formation of natural and artificial induced fractures.In addition,sedimentary rock conditions control the preservation condition of shale gas,and conditions,such as well-enclosed roof and bottom plate or partition,are important preservation conditions for shale gas enrichment.Principally,high-yield wells are in areas with a pressure coefficient greater than 1.2.Therefore,formation overpressure is an important condition for large scale production of shale gas.This study concluded that the section at the bottom of the‘Long 1st section’in southern Sichuan is a favorable exploration and development area,owing to high organic matter content(TOC>3%),continuous thickness~30~50 m,developed lamellae,abundant organic matter pores,brittle mineral content greater than 50%,ultra-high pressure strata,and suitable preservation conditions.Thus,the large scale production area of shale gas in the Longmaxi formation in southern Sichuan is located in the deep shelf sedimentary formation in a structurally stable area.