Study Of The Ordovician Reef Development And Reservoir Characteristics In The Southern Margin Of Ordos Basin

The scale of Ordovician reef development in the southern margin of the Ordoshas been confirmed by a large number of previous studies. As an important marinecarbonate reservoir types, reef reservoirs because of the excellent reservoir propertiesand a number of successful exploration precedents and has important researchsignificance.Based on the basis of previous studies, this article aims at the reef of theOrdovician in the southern margin of Ordos. By outcrop field reconnaissance, drillingcore observation, combined sample test data, this paper systematically studied theReef-related layers and distribution of the Ordovician reefs in the southern margin ofOrdos. Based on the anatomy of typical reefs, the distribution range of organisms andthe rock types are defined. Combined with the analysis of reef layers depositedsedimentary facies and tectonic evolution, the formative Pattern of the reefs in thesouthern margin of Ordos is summarized. Through the observation of thin sections,porosity and permeability test analysis, the reservoir space types, the main diagenesis,porosity evolution and reservoir characteristics of the reef reservoir are defined.Ordovician reef in the southern margin of the Ordos mainly developed in the lateOrdovician Pingliang and Beiguoshan Formation. Through field investigation in thesouthern margin of the basin,13locations of reef development are defined. Theymainly distributed in the region to the north of Weihe,south to Pinliang, fromLongxian to Qishan, Jing Yang, Chunhua, Fuping, Tongchuan area, including coredrilling discovered in Chun2wells and Xuntan1well.11locations fund on themeasured sections, which Pingliang group (or equivalent level position)8locations,Beiguoshan group3locations. The most common reef organisms are corals, spongesand layer hole followed by insects, and some blue-green algae. Attached to the reeforganisms contains brachiopods, gastropods, trilobites, crinoids, ostracods, etc. Themain rock types of reef including biological skeleton rock, rock bonding, barrier sedimentary rocks, bioclastic limestone, limestone and other algal debris. Reeforganisms have obvious social characteristics, biological and various beach facies aresparry calcite cement, belonging to the deposition of high-energy environment.By typical reef anatomy, combined with drilling core observation, sedimentarymicrofacies of reef formations are analyzed, their distribution and sedimentaryevolution is clear, developmental evolution of reefs are summarized. Late Ordovicianin the southern margin of the basin is a marginal sea rimmed platform, characterizedby a combination of developmental platform margin reef. By which the ancient reeflocation can be divided into the platform inside reef, the platform margin reef,platform edge barrier reef, which the inside and the platform edge mainly hummockybarrier reef, the reef can be further divided into reef base, reef core, reef cover,reef-wing and other microfacies. Combined with the analysis of reef layers depositedsedimentary facies and tectonic evolution, the formative Pattern of the reefs in thesouthern margin of Ordos is summarized.Through the reef reservoirs sample microscopic analysis of thin sections andphysical tests, this paper demonstrated that the main types pore of reservoir areintergranular pore, dissolved pore and dissolved seam. Physical characteristics of thereservoir is showed low porosity and low permeability reservoir. Development of thereservoirs is obviously controlled by sedimentary facies and diagenesis. Platformmargin Beach and reef in the energy platform margin are favorable microfcies, pointbeach and reef in open platform followed.Constructive factor of the reservoir development is the dissolution and partialdolomitization, but compaction, filling, cementation and recrystallization are greatlydamaged the properties in reef reservoir.