Dolomitization of the lower cretaceous carbonate reservoir in the Euphrates Graben,Syria

Results of this study are based on core materials description,thin sections,Cathodoluminescence(CL),and Scanning Electron Microscope(SEM)examinations.The Lower Cretaceous over the Euphrates Graben area was characterized by carbonate sedimentation in shallow marine environments.The low energy lagoonal to inner shelf sediments of the Judea Formation includes micritic mudstone to wackestone texture,dolomitic limestones and dolostones.Two types of dolomites recognized in the carbonates of the Judea Formation,the preserving microcrystalline dolomites which commonly founded in the partially dolomitized micritic limestones,and the destructive coarsely crystalline dolomites which commonly founded in the dolostones and dolomitic limestones.Petrographic examinations indicate that the preserving microcrystalline dolomites represent subtidal cycles developed in a shoal to open marine depositional environments,they probably formed under conditions of the shallow burial diagenesis.The destructive coarsely crystalline dolomites may develop in more basinward,open marine environments under conditions of the deep burial diagenesis that accompanied by rising in temperature,pressure,and burial depth.It is believed that evolution of the diagenetic history of the Judean Formation sediments occurred in two diagenetic stages;the shallow burial diagenesis,and the deep burial diagenesis.Compaction processes,early fracturing,micritization,early calcite,and the early phase of dolomitization were part of the multiple diagenetic alterations during the shallow burial diagenesis.The deep burial diagenesis was marked by dissolution,late stage of dolomitization and replacement,mechanical and chemical compaction,and the late calcite precipitation.It is believed that dolomitization of the Judea Formation carbonates in the Euphrates Graben has contributed to improving the reservoir properties by increasing the porosity and thus the permeability.

Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target ofhydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographicallyexaminations, and petrophysical evaluations were obtained to determine enhancement ofthe porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of theJudea formation are subdivided into two main zones;the upper “limestone zone” is micritic limestonesdominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones anddolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones andwackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and earlyfabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted information of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization ofthe “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures,while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanismunder high temperature and pressure conditions. The “limestone zone” is characterized by the poorestreservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawaterdolomitization did not significantly enhance the porosity, while the burial dolomitization contributes toenhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associatedwith dolomitization played as major factors in porosity enhancement of the dolostones and dolomiticlimestones and serving as pathways for hydrocarbon migration. Dissolution processes increased theporosity and more permeability unless they are filled with the precipitated dolomite and/or calcite.Calcification had significant effects on the porosity by blocking the cavities and channels and decreasedthe effective pore volume.

Sedimentary diagenesis and pore characteristics for the reservoir evaluation of Domanik formations(Semiluksk and Mendymsk)in the central part of Volga-Ural petroleum province

The Upper Devonian Semiluksk(Domanik)and Mendymsk carbonate successions have been identified as the most promising unconventional oil and gas deposits in Russia's Volga-Ural petroleum province.Three lithofacies were identified:limestones,organic matter-rich siliceous carbonate,and dolomitic limestones/dolomite.Their sediments are thought to have accumulated in the relatively deep-sea shelf depositional environment.According to X-ray analysis,calcite is the most common mineral phase in the limestones.The siliceous carbonate rocks'major mineral phases were identified to be quartz,followed by calcite.The rocks have relatively poor reservoir qualities,with porosity ranging from 0.1 to 5%and permeability ranging from 0.01 to 10 mD.Petrographic examination of thin sections reveals four major types of pores:1.interparticle pores,which are abundant in all of the examined lithofacies;2.intercrystalline pores formed between dolomite crystals and the very fine components of the siliceous carbonate rocks.3.vugs as a result of leaching,and 4.fracture as a result of tectonic and/or diagenetic events.Plotting the obtained data from the petrographically examined thin sections on the ternary pore plot diagram resulted in the identification of six pore facies and their related diagenetic patterns.Pore facies 1 depicts the pores that formed during the depositional process.Pore facies 4 displays moldic pores.The vugs are represented by pore facies 6.Pore facies 2 is a mixture between pore facies 1 and pore facies 4.Pore facies 3 is a mixture between pore facies 1 and pore facies 6.Pore facies 5 is a mixture between pore facies 4 and pore facies 6.Comparing the petrographic results and the identified pore facies show that pore facies 6 have touching pores and they played the most important role in improving the reservoir quality.The pore facies 4 which have separate pores have the most influence in the reduction of the reservoir quality.The diagenetic origin secondary pores are the most pores identified in the studied carbonate section.This reflects the strong modification of the pores by the diagenetic overprinting.The massive dissolution that forms the touching vugs,and the dolomitization process are the most diagenetic processes that affect the pore system within the studied rocks.