The Vertical Distribution Characteristics And Genesis Of Dawsonite In Pelitic Siltstone With Foraminifers

Dawsonite is a white sodium-and-water-containing aluminum carbonate mineral that requires a high CO₂ partial pressure and a sodium aluminosilicate-rich solution in formation.It is generally accepted that dawsonite is a tracing mineral of CO₂ migration and accumulation.Dawsonite has been identified insandstones of CO₂ gas reservoir in the study area.Studies have shown that the type of stone occurring in dawsonite is mainly muddy siltstone which containingforaminifera.The study will determine the new occurrence of dawsonite development in muddy siltstone containing foraminifera,enrich the rock association occurring dawsonite,expand the depth of the development of dawsonite in the stratum and provide the evidence of geological examples for the depth range of stable existence of dawsonite in the stratum.Through the observation of the casting thin sections with the polarizing microscope and combined with the analysis of the contents of mineral components with the X-ray diffraction,the analysis of the major and trace elements with the X-ray fluorescent spectrometry as well as the analysis of carbon and oxygen isotope in carbonate minerals,this paper studiedthe petrologic feature such as rock type and diagenetic symbiosis sequenceand the geochemical characteristics?carbon and oxygen isotopes etc.?from the Pearl River Formation to the Wenchang Formationreservoir in Liwan sag of Pearl River Mouth Basin.Based on the characteristics of dawsonite developed in the researched area and a horizontal comparison with dawsonite naturally occurring throughout the world,this paper studied the carbon source and ion source of the dawsonite occurring in the researched area,and finallyexplored the formation ofdawsonite within the researched area.The petrographic research showed that the rock types in the stratum of the Pearl River Formation were mainly muddy siltstone containing foraminifera,and dawsonite mainly occurred in it.The burial diagenesis within the researched area experienced a CO₂ gas charging and an oil-gas charging with dawsonite as the “tracing mineral” of CO₂ charging.The authigenic minerals and their autogenic sequences were: clay cladding-authigenic kaolinite-secondary quartz-siderite-pyrite-dolomite-calcite/ferrocalcite–dawsonite–ankerite.Oil-gas charging occurred after the formation of calcite/ferrocalcite,and the CO₂ gas charging occurred later.Geochemical studies have shown that the researched area belonged to continental deposition,the sedimentary environment was a weak reduction environment,and the depositional environment in the stratum of the Pearl River Formation was in the stage of reduction-oxidation.The results of carbon and oxygen isotope analysis showed that the ?¹³C?PDB?value of dolomite,calcite or dawsonite and the ?¹³C?PDB?value of equilibrated CO₂ gasshowed strong inorganic mantle source isotopic characteristics.It is noteworthy that the dawsonite is closely linked to the spatial distribution of the CO₂ gas reservoir and is concentrated in the bottom of the mudstone or argillaceous siltstone cover.This area is also the ultimate rich area after CO₂ enters the reservoir area.The CO₂ fluid with high gas saturation and large dissolved load after the CO₂ enrichment has a low p H value.Aluminosilicate minerals dissolve in such an acidic environment and release the cations,and then combine with sufficient carbon source to produce dawsonite under high CO₂ partial pressure.Carbon and oxygen isotope analysis showed that CO₂ in the gas reservoir was homogenous as the CO₂ in the formed CO₂ dawsonite and both of them were inorganic mantle source.Most of the dawsonite developed in the researched area was in the form of radial or hairy form in the body cavity of the foraminifera and was concentrated in the depth range of 3,100³,300 m,the longitudinal distribution was in thenormal distribution trend and reached the peak at 3,210 m,and the temperature range of the stratum was 180.42¹92.06?.Compared with the dawsonite naturally developed around the world,the dawsonite developed within the researched area had the deepest burial depth and the highest temperature of stable existence was the highest.Na⁺ and Al³⁺ in the developed dawsonite were mainly derived from the CO₂ fluid filling,and the cations released from the dissolution of the aluminosilicate minerals from the Zhuhai Formation to the Wenchang Formation upwelled with the CO₂ fluid and provided the materials source for the development of dawsonite.In addition,the alkaline cation within the cavity wall or cavity of the foraminifera was also the ion source of dawsonite.Finally,establishied a formation model of dawsonite in the muddy siltstone containing foraminifera.