桂中地区泥盆纪碳酸盐岩沉积特征与孤立台地礁滩演化

Sedimentary features of carbonates and evolution of isolated platforms in Devonian in Guizhong area

桂中坳陷是在加里东运动基础上形成的晚古生代海相大型沉积坳陷,其后期构造稳定,为具有较好潜力的勘探新区。其中,该区泥盆系地层深埋地腹,为主要勘探目的层系和近期研究的热点。在总结前人研究的基础上,运用野外露头剖面观测、测录井、地震相分析等技术手段,建立了本区泥盆系层序地层格架,由下而上划分为3个三级层序:SS1、SS2、SS3,完成了桂中地区泥盆系地层与地震层序格架的统一。研究了本区泥盆纪沉积特征和沉积演化史,分析认为桂中坳陷经历加里东期隆升后,在海西早期(D-C)的拉张背景下,演变为'台-盆'相间的沉积格局,期间沉积多个孤立台地。孤立台地边缘发育有生物礁滩,发育时间从中泥盆世早期一直延续至中泥盆世晚期,首次提出该地区孤立台地礁滩发育具有垂向上'三种五期'叠置加积的特征。其中,层序SS1沉积期(早泥盆世晚期-中泥盆早期)为孤立台地礁滩发育的奠基-发展期。层序SS2沉积期(中泥盆世中期-中泥盆世晚期)为孤立台地礁滩发育的繁盛期。层序SS3沉积期(晚泥盆世)为孤立台地礁滩发育的消亡期。

Guizhong depression, which is a large scale Late Paleozoic marine residual basin that formed basically in the stable craton of Late Paleozoic to Mesozoic, have excellent requirements of oil ＆ gas accumulation and exploration potential. Although there is no significant breakthrough in hydrocarbon exploration at present time, it is one of the favorable areas for oil ＆ gas exploration in South China marine deposits because of its steady later structures, dominant uplifting and subsiding movements, weak faulting and compressing deformation and excellent reservoir formation conditions. Based on previous studies, the sequence litho-paleogeographic and sedimentary evolution of the host area in Devonian were analyzed by means of field inspection, well logging, seismic facies analysis and microscope observation in this paper. In order to study the sedimentary features and evolution of isolated reef in Devonian in host area, the following aspects are described in this paper： （1） The general geological conditons of the host area. After the Caledonian movement, it began to be pulled apart in the Devonian and continued to develop in the whole of the Late Paleozoic. Controlled by the contemporaneous faults, isolated carbonate platforms of varying size developed in the deep-water basin. Thus a special paleogeographic framework of ＂platform-basin-hill-trough＂ formed in the host area. （2） The classification of the sequence stratigraphy in Devonian in host area. The Devonian in host area can be divided into 3 third-level sequence： SS1, SS2 and SS3. （3） The research on the litho-paleogeography in Devonian in host area. According to drawing method of sequence litho-paleogeography, taking the third-level sequence as combilation unit. Five litho-paleogeographic maps were drawn, which manifest sedimentary framework of different stage of Devonian in host area. （4） The characteristics and evolution of isolated reef in Devonian in host area. Devonian reef is developed in the host area, and drilling data of well DS1 ＆ GZ1 support the opinion that high quality biolithite and dolomite reservoirs do exist in the host area. There are 3 types reservoirs of reef, which is identified the five stage in the growth of the isolated carbonate platform＇s reef in host area. Based on this study, it worked out that Guizhong depression turned into the sedimentary framework which was platforms and basins between them in the background of Early Hercynian extensional movement after Caledonian up warping. With the characteristics of three types and five stages reefs growing up in vertical direction, several isolated carbonate platform＇s reefs were Distributed in the host area, which generated from the early period to the late period of Middle Devonian.