Journal of Palaeogeography（Chinese Edition）-Channel: SPECIAL ISSUE ABOUT “DOLOSTONE”

Precambrian marine dolomite cements: review and perspectives

NING Meng, LUO Jingru, CHEN Jiale, ZHONG Yijiang, WEN Huaguo — 2025-01-20 09:48:11.007

Marine dolomite cement is widely developed in the Precambrian era,whereas its occurrence in the Phanerozoic is scarce. Precambrian marine dolomite cement has attracted extensive attention in recent years due to its diversity,complex origins,and potential implications for Precambrian seawater chemistry. Research on Precambrian marine dolomite cement mainly focuses on two aspects: one is the formation mechanism through mineralogical analysis,and the other is the investigation into the unique Precambrian‘aragonite-dolomite sea' conditions as reflected by marine dolomite cement. The origin of marine dolomite cement incorporates both primary precipitation and mimetic dolomitization. However,due to a lack of reliable geochemical evidence,there is ongoing controversy in this field. In addition,both the precipitation mechanism of the primary dolomite and the corresponding seawater chemical conditions remain unclear. The identification of precursor minerals for mimetic dolomite is controversial. Based on previous studies,it can be summarized the spatial and temporal distribution,petrological and mineralogical characteristics,and geochemical characteristics. Subsequently,the current research status on the formation mechanism of marine dolomite cements from three aspects,including primary precipitation,secondary replacement,and growth assemblage morphology. Finally,it can be proposed that three aspects should be enhanced: (1)The genesis of diverse types of marine dolomite cements should be investigated based on the principles of mineral nucleation and growth theory,combining with micro area or in-situ geochemical analysis;(2)he specific contribution of microorganisms to the formation of marine dolomite cements requires assessment;(3)To gain a comprehensive understanding of the evolution of Precambrian seawater properties,exploration of the origin of marine dolomite cement is imperative.

Reservoir characteristics and main controlling factors of ultra-deep dolomite in the 4th Member of Sinian Dengying Formation in Penglai area,Sichuan Basin

2025-01-20 09:48:11.007

The Dengying carbonate reservoirs, as one of the primary producing units of ultra-deep hydrocarbon resources in the Sichuan Basin,is required for further studies on their characteristics and controlling factors. This paper focuses on the ultra-deep dolomite of the fourth member of the Dengying Formation in the recently explored Penglai area of the Sichuan Basin. The study examines the reservoir's petrological characteristics,pore spaces,classification of reservoir types,physical properties,diagenetic evolution,and the main controlling factors of the reservoirs. The analysis is based on field outcrop profiles,drilling cores,microsections,cathodoluminescence(CL)imaging,scanning electron microscopy (SEM) data,and physical property test results. The findings are as follows: (1)The primary lithologies of the fourth member of the Dengying Formation in the Penglai area include dolo-thrombolite,dolo-stromatolite,dolo-spongiomicrobialite,and sand-debris dolomite. The main reservoir spaces consist of primary framework pores and secondary dissolution pores formed by subsequent dissolution processes. (2)Fractured reservoirs, primarily composed of dolo-thrombolite and dolo-stromatolite reservoirs with partial bedding pore development, typically exhibit favorable physical properties. The average porosity can exceed 4%,with the average permeability of the former reaching 2.43×10^-3 μm²,while the average permeability of the latter is only 0.35×10^-3 μm². (3)A comprehensive analysis of diagenetic evolution reveals that the microbial carbonate reservoirs of the fourth member of the Dengying Formation have undergone a sequence of stages: sedimentary-quasi-symbiotic,supergene,shallow-to-medium burial,and deep-to-ultra-deep burial stages. (4)Analysis of reservoir distribution patterns in the study area indicates that the reservoir distribution weakens from the platform interior to the platform edge. Overall,the ultra-deep dolomite in the fourth member of the Dengying Formation in the Penglai area follows a ternary reservoir model characterized by “sedimentary facies-fault-dissolution.” This study provides key insights into the characteristics and controlling factors of the ultra-deep dolomite reservoirs in the Penglai area of the Sichuan Basin,offering significant guidance for future ultra-deep oil and gas exploration in the region.

Ultra-deep dolomite types and their reservoirs potential of the Ordovician Yingshan Formation in Shunbei area,Tarim Basin

2025-01-20 09:48:11.007

Dolomites of the Ordovician Yingshan Formation,which exhibit significant exploration potential,are found in the Shunbei area of the Tarim Basin. However,there is no systematic explanation on the genesis and physical property variations of the stratified dolomite in this region. Based on detailed petrology observation,carbon and oxygen isotopes,rare earth element analysis,carbonate U-Pb chronology dating,and porosity and permeability testing under different stress states,this study discussed the types and genetic mechanisms of the inner stratified dolomites of the Yingshan Formation. Furthermore,it provides a preliminary evaluation of the differences in physical properties among dolomites of various origin. The findings can be summarized as follows: (1)The relative development of dolomites in the lower part of the Yingshan Formation located in the northern region of Shunbei area primarily consists of two types of stratified dolomites,including the buried pressure-dissolved dolomite(Class I)and the penecontemporaneous seepage reflux dolomite(Class Ⅱ). (2)The Class I dolomites,which are predominantly distributed in low-energy sedimentary facies,have their origin in burial dolomitization. This is attributed to the genetic mechanism of clay mineral transformation,driven by the formation of stylolites during the burial process. These dolomites are relatively more developed in the central part of the Shunbei area. (3)The Class II dolomites are characterized by penecontemporaneous seepage reflux dolomitization in sedimentary facies that display high geomorphology and relatively high energy. These dolomites are primarily distributed and well developed in the western and southern Shunbei area. (4)The measured physical properties of Class Ⅰ dolomites(φ: 0.3%~1.0%;K: 0.003×10^-3~0.074×10^-3 μm²)are lower than those of Class Ⅱ dolomites(φ: 0.4%~2.0%;K: 0.009×10^-3~0.055×10^-3 μm²). However,Class I dolomites can enhance the ratio of dolomite-limestone interbedding,and there are fractures to transform into reservoir potential. This study is of significant reference value for ultra-deep dolomite oil and gas exploration in the Shunbei area of the Tarim Basin.

Amplitudes of Earth's orbital parameters and fluctuations of deep-water dolomitization

ZHANG Kuihua, ZHANG Pengfei, MIAO Zhuowei, ZHANG Qian — 2025-01-20 09:48:11.007

Due to the evident correlation between fluctuations in dolomitization intensity in Phanerozoic geological strata and significant geological events throughout Earth's history,it is increasingly important to understand the mechanisms underlying these fluctuations in relation to long-term magnesium and carbon cycles in terrestrial and marine environments,atmospheric processes,and climate change over extended periods. In this study,time series analysis and coupling analysis are analyzed based on high-resolution X-ray diffraction data to reveal the slope amplitude modulation cycle recorded by the deep-water dolomite cycle of the Shahejie Formation in the Bohai Bay Basin. Combining the Quaternary dolomite cycle in Van Lake,Turkey,with the dolomitization events during the Permian-Triassic period,we discussed the coupling relationship between the fluctuations in deep-water dolomitization intensity over the Phanerozoic eon and the amplitude of various Earth orbital parameters across different time scales. Building on the analysis of the salinity evolution characteristics of the Sha 3 submember,it is proposed that the amplitude-modulation effect of Earth's orbital parameters may influence microbial ecological stress and the fluctuations in temperature and pH of fluids through changes in climate and environmental factors. This,in turn,plays a significant role in regulating orbital deep-water dolomitization intensity during the Phanerozoic eon. This study provides a novel perspective for the exploration of the “dolomite problem”.