川中龙岗地区下侏罗统自流井组大安寨段湖相混合沉积及其致密油勘探意义

Mixing of Lacustrine Siliciclastic—Carbonate Sediments and Its Significance for Tight Oil Exploration in the Daanzhai Member, Ziliujing Formation, Lower Jurassic, in Longgang Area, Central Sichuan Basin

下侏罗统自流井组大安寨段是川中地区的主力产油层之一,通常认为储集体为高能介壳滩相的"纯"灰岩。本文基于露头、岩芯、薄片、扫描电镜等观察,认为川中龙岗地区自流井组大安寨段为一套典型的湖相混合沉积,包括狭义的混积岩和广义的混合层系。狭义混积岩成分以碳酸盐为主,由碳酸盐、陆源碎屑和泥质以不同比例混积而成,包括含砂泥质介壳灰岩、含泥砂质介壳灰岩、含介壳含泥钙质砂岩、含介壳含砂钙质泥页岩等岩石类型。广义混合层系主要为介壳灰岩、泥页岩、砂岩与狭义混积岩等构成的互为夹层或近等厚互层。混合沉积类型主要有间断混合、原地混合和相混合3种,分别受控于风暴作用、水动力条件和优势相。由于后期强烈的压实作用,原生孔隙消失殆尽,无论是较纯的介壳灰岩还是混合沉积都极为致密,其储集空间主要为溶蚀孔洞、溶蚀微孔、微裂隙和构造裂缝。除了"纯的"介壳灰岩外,由介壳和粉砂—泥质混积而成的泥质介壳灰岩和含介壳泥页岩由于微裂隙相对发育,部分物性好的层段也可以作为储层。上述结果和钻测成果暗示:临近烃源的低能介壳滩和介壳滩缘等混积相的粉砂—泥质介壳灰岩和含介壳泥页岩是一种广泛发育的潜在储集体,这对于致密油勘探而言尤为重要,应该引起重视,建议进行试点勘探。

The Daanzhai Member of the Lower Jurassic Ziliujing Formation is one of the major oil production horizons in Sichuan Basin, where ＂pure＂ limestone from high-energy shelly shoal of shallow lake facies has been usually viewed as the only kind of reservoir rock in previous studies. Based on outcrop surveying and borehole core observation as well as thin section and SEM （ scanning electron microscopy） analyses, we propose in the paper that the Daanzhai Member of Ziliujing Formation is a typical lacustrine depositional sequence of mixed siliciclastic-- carbonate sediments in Longgang area, central Sichuan Basin, which contains two levels of implication. In narrow sense, the mixed siliciclastic--carbonate rock means mixing of major lacustrine carbonate with minor terrigenous clastics and/or clay in various proportion. Main rock types consist of shelly limestone, sandy/silty/argillaceous shelly carbonate, calcareous shelly sandstone/siltstone/mudstone/shale, etc. In broad sense, the mixed depositional sequence implies alternating layers of shelly limestone, mudstone/shale, sandstone/sihstone and mixed siliciclastic--carbonate rock with unequal thicknesses. In general, there are three ways of mixing： punctuated mixing, in-situ mixing and facies mixing, which are respectively controlled by storm wave action, hydrodynamic condition and predominant facies. Both relatively pure shelly limestone and mixed siliciclastic--carbonate rocks are extremely tight. Almost all primary porosity has disappeared because of intense compaction since burial. The present pore spaces are largely composed of dissolution pore, dissolution micropore, microfracture and structural fracture, etc. In addition to ＂pure＂ shelly limestone, some silty/argillaceous shelly limestone and shelly siltstone/ mudstone/shale with relatively good porosity and permeability could also serve as one kind of effective reservoir due to better-developed microfractures within them. These observations and drilling results imply that the silty/ argillaceous shelly limestone of low-energy shelly shoal and surrounding shelly sihstone/mudstone/shale adjacent to hydrocarbon generating center are extensively developed and potential reservoir. This is particularly important for tight oil exploration, which should be paid special attention to. Therefore, a pilot exploration is recommended to be carried out.