塔河油田塔深6井蓬莱坝组硅质岩成因及其地质意义

塔里木盆地下奥陶统蓬莱坝组普遍为局限台地相沉积,塔河地区最为典型,前期数口钻井已揭示塔河地区蓬莱坝组以中-厚层粉-细晶白云岩为主。而其西南部的塔深6井揭示蓬莱坝组岩性差异较大,主要为砾屑白云岩、深灰色角砾状硅质和浅灰色层状硅质岩组合,其中发育一套近60 m浅灰色层状硅质岩。通过岩心观察、常规岩矿、测井和地震相分析,识别出塔深6井蓬莱坝组沉积背景为局限台地内的局部负低洼区,南北长约300 m,东西宽约200 m,台洼高差可达上百米,发生滑塌而产生角砾状硅质和砾屑白云岩的混杂堆积,并形成浅水造硅生物建造的均匀层状硅质岩,不同于周边典型的局限台地厚层白云岩组合。层状硅质岩具有中孔、低渗特征,储集空间主要为直径1～6μm的密集微孔,见菌丝等古微生物痕迹。由此,确定塔深6井蓬莱坝组硅质岩主要为由沉积地形诱发的机械垮塌破碎和浅水-半深水生物成因,对于丰富和深化塔里木盆地奥陶系沉积演化和储层成因具有重要意义。

Genesis and geological significance of siliceous rock in Penglaiba Formation in Well Tashen 6, Tahe oilfield

Restricted platform facies is common in the Penglaiba Formation of the Lower Ordovician in the Tarim Basin. This is especially true in Tahe area, where the early drilling data reveal that the Penglaiba Formation is mainly composed of moderate-to-thick-bedded very fine to fine crystalline dolomites. However, drilling data of Well Tashen 6 in the southwest of Tahe area reveal that the Penglaiba Formation is quite heterogeneous in terms of lithology in this area and is mainly composed of intraclast dolostones, dark gray brecciated siliceous rocks and light gray laminar siliceous rocks, within which a set of light gray laminar siliceous rocks of nearly 60 m in thickness was developed. Through core observation, conventional thin section examination, well logging and seismic facies analyses, we identified that the sedimentary setting of Penglaiba Formation in Well Tashen 6 is locally low within the restricted platform, about 300 m long from north to south and 200 m wide from east to west, with a higher relief up to hundreds of meters. Therefore, the collapse therein could produce a mixed accumulation of brecciated siliceous rocks and intraclast dolostones, and form homogeneous laminar siliceous rocks constructed by shallow-water silicogenic organisms, quite different from the thick-bedded dolomite assemblage of the surrounding restricted platform typical in the area. The laminar siliceous rocks are characterized by medium porosity and lower permeability as well as reservoir space dominated by massive micropores with a diameter of 1-6 μm. And traces of ancient microorganisms such as hyphae are seen therein. In conclusion, it can be confirmed that the siliceous rocks in the Penglaiba Formation in Well Tashen 6 are mainly of mechanical collapse origin induced by the special topography and shallow-to-semi-deep water biogenesis. This is of great significance to enriching and deepening understanding of the Ordovician sedimentary evolution and reservoir genetic mechanism in the Tarim Basin.