准噶尔盆地玛湖凹陷二叠系—三叠系构造层序格架与沉积叠合演化

玛湖凹陷是准噶尔盆地西北部的重要富烃凹陷和大型地层岩性油藏勘探区域,形成多层系的大型地层岩性油气藏。为了查明该区油气分布规律,应用构造层序分析法,通过岩芯观察和地震、录井、测井等资料,对二叠系—三叠系油气主要赋存层系进行研究。结果表明:玛湖凹陷二叠系—三叠系存在多个大型地层不整合面,可划分为下二叠统、中二叠统、上二叠统—三叠系等构造层序。二叠纪到三叠纪,玛湖地区经历了断陷、坳陷和大型坳陷斜坡三大演化阶段,形成了三大构造层序的叠置。下二叠统为伸展构造背景下形成的箕状结构构造层序,北部地层厚、南部地层薄,在北深南浅的箕状结构背景下沉积,凹陷区为分布广泛的细粒混积岩,为优质烃源岩建造,朝向斜坡区岩性逐渐变为砂岩、砾岩; 中二叠统为坳陷构造背景下形成的中部厚、周缘薄的构造层序,发育盆地边缘扇三角洲,表现为盆大、水浅,盆地边缘扇体有限,盆内砂泥互层沉积; 上二叠统—三叠系,玛湖地区处于准噶尔大型坳陷盆地的西北部斜坡区,在浅水、缓坡和湖侵背景下形成大型退覆式扇三角洲沉积,砂体呈纵向叠置、横向连片分布。三大构造层序的纵向叠置,形成了该区多层分布地层-岩性或构造-岩性油藏。

Structural Sequence Framework and Sedimentary Superimposed Evolution of Permian-Triassic in Mahu Sag of Junggar Basin,China

Mahu sag is an important hydrocarbon-rich sag and a large stratigraphic lithologic reservoir exploration field in the northwestern Junggar Basin, forming a large stratigraphic lithologic reservoir with multiple layers. In order to find out the distribution law of oil and gas in this area and provide geological basis for the next exploration deployment, based on core observation and comprehensive analysis of seismic, logging and other data, the structural sequence analysis method was applied to study the main oil and gas reservoir series from Permian to Triassic. The results show that there are many large stratigraphic unconformity surfaces from Permian to Triassic in Mahu sag, which can be divided into Lower Permian, Middle Permian, Upper Permian-Triassic and other structural sequences. From Permian to Triassic, it experiences three major evolution stages of fault depression, depression and large depression slope, forming the superposition of three major structural sequences. The Lower Permian is a dustpan structural sequence formed under the extensional tectonic background; the northern stratum is thick, and the southern stratum is thin; it is deposited under the dustpan structure background of deep north and shallow south; the depression area is widely distributed fine-grained mixed rock, which is constructed of high-quality hydrocarbon source rock, and gradually becomes sandstone and conglomerate towards the slope area. The Middle Permian is a structural sequence with thick central and thin peripheral margins under the tectonic background of depression; the basin margin fan delta is developed, which is characterized by large basin, shallow water, limited basin margin fan body, and interbedded sediment of sand and mud in the basin. In Upper Permian-Triassic, Mahu area is located in the northwest slope area of Junggar large depression basin; under the background of shallow water, gentle slope and lacustrine transgression, the large retrogressive fan delta deposits are formed, and the sand bodies are vertically superimposed and horizontally distributed. The vertical superposition of the three major structural sequences forms the multi-layer distribution of stratum-lithologic or structure-lithological reservoirs in the area.