A numerical model for porosity modification at a sandstone-mudstone boundary by quartz pressure dissolution and diffusive mass transfer

In many sandstones quartz cements significantly reduce porosity. The origin of these cements is often unclear. This paper investigates a possible mechanism for the generation of silica for quartz cement by pressure dissolution in interbedded mudstones. Theoretical models of quartz pressure dissolution, including the effects of silica precipitation kinetics, show that the concentration of dissolved silica in the pore fluids of a compacting sediment increases with decreasing grain size and silica precipitation constant. Quartz precipitation is strongly inhibited by the presence of small amounts of clay within a sediment, suggesting that siltstones and quartz-rich mudstones which are undergoing pressure dissolution may act as a source of dissolved silica for export to nearby, coarser sediments. A computational model for the diagenetic modification of a sandstone-mudstone interface due to pressure dissolution is described. Both sandstone and mudstone layers are assumed to be actively compacting by pressure dissolution, and mass transport by molecular diffusion is considered. As quartz precipitation in the mudstone layer is relatively slow compared to that in the sandstone, significant amounts of dissolved silica become available in the mudstone, and may be exported into the adjacent sandstone. In the absence of pore-fluid advection, this may result in the formation of extensive secondary quartz within the sandstone, close to the interface. The volume of silica exported from the mudstone is limited by the length scale over which diffusion through the mudstone is effective. This is typically 3–5 m. The volume of silica available therefore suggests that extensive porosity modification within the adjacent sandstone can only occur close to the mudstone. Thus it is possible that thin sandstones could become cemented by slow diffusive transfer of silica, but that in thicker sandstones the silica may become dispersed by pore-fluid advection.     

大港滩海区第三系湖相混积岩的成因与成岩作用特征

大港滩海第三系沙河街组一段下部分布的混积岩主要为陆源碎屑质-碳酸盐混积岩, 其次为碳酸盐质-陆源碎屑混积岩和含碳酸盐-陆源碎屑混积岩.其中白水头地区的混积岩发育于辫状河-扇三角洲前缘水下分流河道沉积体系之中,为相缘渐变混合沉积的产物;而马东-马东东地区的混积岩发育于重力流沉积体系之中,为浊流沉积的产物.本区混积岩主要经历了压实作用、胶结作用和溶蚀溶解作用.孔隙流体的化学性质经历了由碱性到酸性,最后又回到碱性的变化历程.其中海绿石、微晶方解石、部分石英溶蚀和连生方解石为碱性流体条件下的共生组合;次生加大石英、油气侵位、溶蚀溶解、高岭石为酸性条件下的共生组合;伊利石、绿泥石、钠长石化、孔隙充填方解石和白云石为碱性流体条件下的共生组合.     

鄂尔多斯盆地高桥地区盒8段砂岩储层致密成因

运用常规薄片、铸体薄片、扫描电镜及恒速压汞等实验手段,对鄂尔多斯盆地高桥地区盒8段储层特征进行研究,并以孔隙演化过程为线索分析了储层致密成因。研究结果表明:盒8段储层主要为岩屑石英砂岩、石英砂岩与岩屑砂岩,成分成熟度和结构成熟度均较高,储层物性差;砂岩分选性好,初始孔隙度整体较高,平均可达34.7%;受岩石组分、埋深及煤系烃源岩酸性流体的影响,压实作用强烈,破坏了大约20.4%的原生孔隙度,胶结作用破坏了其余13.9%的原生孔隙度,原生孔隙几乎被破坏殆尽;早期溶蚀产生的孔隙被压实作用破坏,而晚期溶蚀作用较弱,仅增加了大约3.4%的次生孔隙度,难以大幅度改善储层物性。因此,成岩期原生孔隙被破坏殆尽及次生孔隙形成较少共同导致该区储层致密。     

Effects of phosphate ions on intergranular pressure solution in calcite: An experimental study

Intergranular pressure solution (IPS) is a coupled chemical-mechanical process of widespread importance that occurs during diagenesis and low-temperature deformation of sedimentary rocks. Laboratory experiments on IPS in halite, quartz, and calcite have largely concentrated on the mechanical aspects of the process. In this study, we report the effects of pore fluid chemistry, specifically varying phosphate ion concentration, on the mechanical compaction by IPS of fine-grained calcite powders at room temperature and 1 to 4 MPa applied effective stress. Phosphate was investigated because of its importance as a biogenic constituent of sea and pore waters. Increasing the pore fluid phosphate concentration from 0 to 10⁻³ mol/L systematically reduced compaction strain rates by up to two orders of magnitude. The sensitivity of the compaction strain rate to phosphate concentration was the same as the sensitivity of calcite precipitation rates to the addition of phosphate ions reported in the literature, suggesting that the rate of IPS in phosphate-bearing samples was controlled by calcite precipitation on pore walls. The results imply that IPS and associated porosity/permeability reduction rates in calcite sediments may be strongly reduced when pore fluids are enriched in phosphates, for example, through high biologic productivity or a seawater origin. Future modeling of IPS-related processes in carbonates must therefore take into account the effects of pore fluid chemistry, specifically the inhibition of interfacial reactions.     

Relationships among sedimentology, stratigraphy, and diagenesis in the Proterozoic Thelon Basin, Nunavut, Canada: implications for paleoaquifers and sedimentary-hosted mineral deposits

The Thelon Basin, Nunavut, Canada, is host to unconformity-type uranium mineralisation and has the potential to host other economic deposits. The Thelon Formation (ca. 1750 Ma) is composed of thick (meters to tens of meters), poorly sorted, trough cross-bedded conglomerate and coarse-grained lithic arenite beds, and to a lesser extent, well-sorted, medium- to coarse-grained quartz arenite beds. Relatively rare, 1–12 cm thick, clay-rich siltstones to fine-grained sandstone layers punctuate the coarser lithofacies. Based on regional analysis of drill cores and outcrops, multiple unconformity-bounded sequences are defined in this fluvial-dominated sedimentary succession. Stratigraphic correlations are based on detailed lithofacies analysis, distinct changes in fining-upward cycle thickness, and intraformational surfaces (unconformities, transgressive surfaces, and paleosols).Diagenetic and paragenetic relationships vary systematically with sedimentology and stratigraphy of the Thelon and provide a framework for understanding the evolution of fluid-flow systems in the context of basin hydrostratigraphy. Stratigraphic units with well-sorted textures, which lacked clay and unstable framework grains, originally were aquifers (depositional aquifers) during early basin evolution. However, pervasive, early quartz cementation radically reduced the porosity and permeability of these units, occluding pore throats and transforming them into aquitards. Proximal fluvial and alluvial fan lithofacies that contained detrital, mechanically infiltrated, and diagenetic clay minerals and/or unstable detrital grains originally had low permeabilities and only experienced minor quartz cementation. In the deep burial setting (2–7 km), these units retained sufficient permeability to allow diagenetic fluid flow (diagenetic aquifers) as suggested by feldspar dissolution, quartz dissolution, and formation and recrystallization of illite and other diagenetic reactions. Tracing potential diagenetic aquifer and aquitard units across the study area allowed development of a hydrostratigraphic model. In this model, diagenetic aquifers onlap onto, and focused basinal fluids into basement rocks to the east in the Thelon Basin (in the vicinity of the Kiggavik uranium deposit).     

川东建南地区须家河组储层特征及其差异演化过程

川东建南地区须家河组储层非均质性极强,致密化程度极高,为了查明储层差异演化过程,剖析储层差异演化控制因素,运用岩石学和岩石地球化学的多种实验方法,在详细刻画储层岩石学特征的基础上,划分了储层成岩相类型,查明了不同类型砂岩的物性特征和孔隙结构特征,分析了储层的差异性演化过程.研究区砂岩可以划分为4种成岩相类型,即强压实相、溶蚀相、硅质胶结相和强钙质胶结相.溶蚀相砂岩物性最好,强压实相砂岩次之,硅质胶结相和钙质胶结相砂岩物性最差.碎屑组分的差异是造成储层非均质性强和差异性演化的重要原因.塑性岩屑含量的差异,导致早成岩时期压实减孔程度的不同;长石含量的差异,影响了砂岩中溶蚀强度和方解石胶结程度;石英含量的差异,控制了砂岩中硅质胶结作用的强弱;最终,使得储层演化有先有后,空间分布非均质.      

四川盆地须家河组砂岩储层中自生绿泥石的来源与成岩演化

由于在成岩过程中能有效抵抗上覆岩层的机械压实作用,抑制石英在颗粒表面成核的数量,自生绿泥石包膜对四川盆地须家河组砂岩储层的原生粒间孔的保存起到了重要的作用。须家河组早期自生绿泥石物质来源主要有同沉积的富铁沉积物的溶解、河流溶解铁的絮凝沉淀以及相邻泥岩压释水的贯入,主要以新生沉淀形式集中形成于同生期至早成岩早期。成岩中后期,由于下伏富Mg流体沿裂缝系统的持续进入,须2段的自生绿泥石的继续生长得到了充足的物质来源,其MgO的含量随地层深度明显增加,而须4段后期再生长现象不明显。该区自生绿泥石包膜对储层物性的影响通过绿泥石含量与参与孔隙度的关系以及孔隙度分布频率图可以得到较好的评估。从对储层的影响来说,自生绿泥石包膜存在一个最合适的厚度值,超过或低于这个值就会对储层造成负面影响而无法保护储层,该区此最适合的厚度值为5～10 μm。     

Reservoir Porosity Evolution Characteristics and Evaluation of the Jurassic Deep Reservoir from Dibei in Kuqa Depression: Insight from Diagenesis Modeling Experiments Under the Influence of Burial Mode

In this study, using an independently developed diagenetic physical modeling system, we conducted a series of diagenesis physical modeling experiments to simulate reservoir physical property of the Jurassic coarse sandstone from Dibei in Kuqa depression. These experiments reproduced the geological process of early, long-term, shallow burial and subsequent rapid, deep burial, and late tectonic uplift. As the gradual increasing of the simulated temperature, pressure, depth and the continuous strengthening of the compaction, intragranular irregular cracks appeared in quartz, and secondary dissolution could be more easily discovered in feldspar; Porosity, pore throat and K, Na, Ca, Al, Si, Ti, Mn in diagenetic fluid all showed significant evolution and regularity; Calcite, quartz, I/S mixed layer and chlorite were commonly found in modeling samples. At the depth of 3 000~6 500 m, accompanied by strong dissolution, pore diameter and throat diameter reached the maximum, and calcite cementation and quartz overgrowth also increased quickly. At the tectonic uplift stage of 5 000~4 500 m, under the influence of temperature and pressure relief, sandstone porosity began to rebound, and feldspar and rock debris were obviously dissolved under the acidic water from hydrocarbon charging. The formation of effective reservoirs in Western China’s oil and gas basins can be deeply influenced by the factors of dissolution, the process of early, long-term, shallow burial and subsequent rapid, deep burial, and late tectonic uplift, lateral extrusion and cracks induced by tectonic action.     

Diagenesis and reservoir quality analysis in the Lower Cretaceous Qishn sandstones from Masila oilfields in the Sayun–Masila Basin,eastern Yemen

Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD. Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–smectite occur as pore‐filling and pore‐lining cements, which were either accompanied by or followed the development of the early calcite cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition to partial or complete dissolution of calcite cements and unstable grains. Copyright © 2015 John Wiley & Sons, Ltd.     

CO2 production in tar sand reservoirs under in situ steam temperatures: Reactive calcite dissolution

Recovery of highly viscous oil from some of the deeper oil sand deposits of northern Alberta, Canada, is made possible through injection of heat by steam or hot water flooding of the reservoirs. The rise in temperature lowers the viscosity of the bitumen allowing it to be produced. The increase in temperature accelerates the reactions between the matrix and pore minerals of the formation and can produce reaction products which can significantly alter the permeability of the reservoir. If carbonate minerals are present in the reservoir, inorganic CO₂ may also be a reaction product.

The Grand Rapids reservoir consists of relatively clean quartz sand containing 7 wt.% kaolinite, 1 wt.% calcite and a trace of smectite. Core floods of this sand by a neutral NaCl brine at 265°C, 8.2-MPa overburden pressure, 6.0-MPa fluid pressure and a flow velocity of 0.4 pore volumes per hour were used to determine the potential for hydrothermal reactions between clays and carbonate minerals in a natural reservoir sand. Reaction progress was followed by continuous sampling of the production fluids. The produced water was analyzed and the phase chemistry was calculated back to the run conditions using the computer code SOLMNEQF.

Mass-balance considerations on produced total inorganic carbon (TIC) show that calcite broke down very quickly, the maximum in CO₂ production occurring after only one pore volume of fluid had passed through the core. The Ca released from the breakdown of calcite was incorporated in the formation of smectite as was shown by post-run clay mineral analysis by the following unbalanced chemical reaction:

calcite+kaolinite+H₄Si04Ca-smectite+H₂0+CO₂

Silica was supplied by the dissolution of quartz. Silica concentrations analyzed in the production fluid were depressed from those predicted by previously published quartz rate equations because of the rapid rate of smectite synthesis.

These observations were used to formulate the following model for the passage of the first pore volume of NaCl brine through the core. Initially calcite is present throughout the core. As the brine enters the inlet of the core, it equilibrates with calcite. The brine remains in equilibrium with calcite throughout the core as quartz and kaolinite react to form smectite. This model was tested with the computer code PATH.UBC using CO₂ production as a measure of the progress variable ξ. A best fit was achieved to the produced fluid chemistry by varying relative dissolution rates of kaolinite and quatz and varying the suppression of precipitation of certain minerals.     

龙女寺东端须家河组致密砂岩成岩作用及储层形成机理

摘要：致密砂岩作为非常规油气的重要载体,对其致密化成因及储层形成机理研究具有重要意义。利用钻井岩心、薄片观察、阴极发光和流体包裹体等手段,对川中地区龙女寺东端须家河组致密砂岩的成岩作用及储层形成机理进行研究。结果表明：龙女寺东端须家河组遭受了机械压实-压溶作用、胶结作用、交代作用、溶蚀作用和构造破裂作用,机械压实作用和胶结作用是导致砂岩致密化的主要因素,溶蚀作用和破裂作用是储层形成的主要因素;须家河组目前处于中成岩A-B期,经历的流体活动主要有同生成岩期地表水沉淀早期方解石胶结和绿泥石薄膜,早成岩B期压溶作用控制石英Ⅰ期次生加大形成与孔隙流体溶解黏土矿物、长石和易溶岩屑等,中成岩A期大量油气充注促进石英Ⅱ、Ⅲ期次生加大与少量方解石溶解,中成岩B期燕山/喜山运动促进储层形成;储层形成机理主要涵盖残余原生孔的保存和次生孔的形成,有利的沉积环境、溶蚀作用、异常高压和构造破裂作用共同控制储层形成,有利的沉积环境、溶蚀作用和异常高压主要是保护残余原生孔和形成次生孔,构造破裂作用产生大量微裂缝,提高储层渗流能力,并为形成溶蚀扩大孔提供条件。     

Pore water evolution in sandstones of the Groundhog Coalfield,northern Bowser Basin,British Columbia

The succession of sandstone cements in chert and volcanic lithic arenites and wackes from the northern Bowser Basin of British Columbia comprises a record of diagenesis in shallow marine, deltaic, and coastal plain siliciclastic sediments that pass through the oil window and reach temperatures near the onset of metamorphism. The succession of cements is consistent with seawater in the sandstones mixing with acid waters derived from dewatering of interbedded organic rich muds. Sandstone cement paragenesis includes seven discrete cement stages. From earliest to latest the cement stages are: (1) pore-lining chlorite; (2) pore-lining to pore-filling illite; (3) pore-filling kaolinite; (4) oil migration through some of the remaining connected pores; (5) chlorite dissolution; (6) quartz cement; and (7) calcite cement. These seven cement stages are interpreted as a record of the evolution of pore waters circulating through the sandstones after burial. The earliest cement stages, as well as the depositional environments, are compatible with seawater as the initial pore fluid. Seawater composition changed during transport through the sandstones, first by loss of Mg²⁺ and Fe²⁺ during chlorite precipitation (stage 1). Dewatering of interbedded organic-rich mudstones probably added Mg²⁺ and Fe²⁺ to partially buffer the loss of these cations to chlorite. Acids produced during breakdown of organic matter are presumed to have mixed into sandstone pore fluids due to further compaction of the muds, leading to reduction of initial alkalinity. Reduction in alkalinity, in turn, favours change from chlorite to illite precipitation (stage 2), and finally to kaolinite (stage 3). Pore waters likely reached their peak acidity at the time of oil migration (stage 4). Chlorite dissolution (stage 5) and quartz precipitation (stage 6) occurred when pores were filled by these hydrocarbon-bearing and presumably acidic fluids. Fluid inclusions in fracture-filling quartz cements contain petroleum, high-pressure methane, and methane-rich aqueous solutions. Homogenization temperatures from primary two-phase inclusions are consistent with quartz cementation during progressive heating between approximately 100 and 200°C. Following quartz precipitation, alkaline pore waters were re-established, as evidenced by late-stage calcite cement (stage 7).     

成岩体系对致密砂岩储层质量的控制 ——以四川盆地中台山地区须二段为例

为探讨成岩体系对致密砂岩储层质量的控制机理,综合岩心、铸体薄片、阴极发光、X衍射、扫描电镜、包裹体分析、孔渗和三维地震等资料,对四川盆地中台山地区须二段致密砂岩储层成因机制进行了分析。研究发现中台1井须二段主要发育长石石英砂岩、文探1井须二段主要发育岩屑石英砂岩,储集空间主要为长石和岩屑粒内溶孔,孔隙度多<8%,渗透率多<1×10-3 μm2;须二段砂岩经历压实、胶结、溶蚀和交代等成岩作用;中台1井须二段断裂发育,早期石英胶结物主要充填粒间及早期裂缝且成岩流体古盐度多<20%,晚期裂缝多被晚期硅质和方解石胶结物全充填且成岩流体古盐度多>20%;文探1井须二段断裂不发育,主要发育早期方解石和石英胶结且成岩流体古盐度多>20%;中台1井和文探1井均可见次生溶蚀产物,硅质与黏土胶结物多伴生出现,并普遍充填粒内溶孔或粒间孔,表现出次生溶蚀产物短距离迁移特征。研究表明中台1井早期为开放性成岩体系,晚期为封闭性成岩体系,而文探1井主要表现为封闭性成岩体系;开放体系下次生溶蚀产物能通过裂缝向外迁移产生孔隙度净增量,能有效改善储层质量,而封闭体系下次生溶蚀产物不能向外界迁移仍胶结充填于储层孔隙中,仅为储层孔隙空间再分配,储层孔隙度无净增量,质量无明显改善;中台山地区须二段致密砂岩储层受成岩体系演化控制,早期开放体系有利于储层质量改善,晚期封闭体系有利于早期形成储层的保存,裂缝的开启与封闭是影响成岩体系的关键因素。     

Hydrothermal alteration of granite by meteoric fluid: an example from the Carnmenellis Granite,United Kingdom

The interaction of granitic rock with meteoric fluid is instrumental in determining the chemistry of pore fluids and alteration mineralogy in downflow portions of convective groundwater circulation cells associated with many hydrothermal systems in the continental crust. Hydrothermal experiments and a detailed mineralogical study have been carried out to investigate the hydrothermal alteration of the Carnmenellis Granite, Cornwall, UK. Samples of drill chippings from a borehole 2 km deep in the Carnmenellis Granite have been reacted with a dilute Na-HCO₃-Cl fluid in hydrothermal solution equipment at temperatures of 80°, 150° and 250° C and a pressure of 50 MPa, with a water/rock mass ratio of 10, for experiment durations up to 200 days. Fluid samples were analysed for seventeen different chemical components, and solids were examined prior to, and after reaction using SEM, electron microprobe and conventional light optic techniques. Experimental fluids were mildly alkaline (pH 7–8.5) and of low salinity (TDS <800 mgl^–1). Mineral-fluid reaction was dominated by the dissolution of plagioclase and the growth of smectite, calcite (at all temperatures), laumontite (at 150° C), wairakite and anhydrite (at 250° C). Final fluids were saturated with respect to quartz and fluorite. Certain trace elements (Li, B, Sr) were either incorporated into solids precipitated during the experiments or sorbed onto mineral surfaces and cannot be considered as conservative (partitioned into the fluid phase) elements. Concentrations of all analysed chemical components showed net increases during the experiments except for Ca (at 250° C) and Mg (at all temperatures). A comparison of the alteration mineralogy observed in the experiments with that present as natural fracture infills in drillcore from the Carnmenellis Granite reveals that the solid products from the experiments correspond closely to mineral assemblages identified as occurring during the later stages of hydrothermal circulation associated with the emplacement of the granite.     

鄂尔多斯盆地苏77、召51区块山23段储层成岩作用与孔隙演化定量分析

利用扫描电镜、岩石薄片、高压压汞、X衍射等实验及物性分析资料,对鄂尔多斯盆地苏77、召51区块山23段储层基本特征、成岩作用类型及强度、孔隙演化模式进行研究。结果表明：储层主要岩石类型为成分成熟度较高、结构成熟度较低的石英砂岩和岩屑石英砂岩;孔隙类型以溶孔为主,残余粒间孔和晶间孔含量次之。储层在经历中等-强程度的压实,中等程度的胶结以及强烈的溶蚀作用后,处于中成岩阶段B期,局部处于晚成岩阶段。压实作用和胶结作用导致原生孔隙遭到破坏,孔隙度损失分别为1694％和1429％,晶间孔和溶蚀作用分别使孔隙度增加092％和564％,储层物性得以改善。孔隙空间经历原生孔隙减少→次生孔隙发育→次生孔隙减少→混合孔隙并存的复杂演化过程。     

Fluid Flow History in Lower Triassic Bulgo Sandstone,Central Sydney Basin,Australia

INTRODUCTIONThe Sydney basin(Fig.1) forms the southernm ost partofthe Sydney- Gunnedah- Bowen basin system (Bembrick et al.,1973) .It started as a rift and evolved into a foreland basin inMiddle Triassic with sediments lapping unconformably on thedeformed and metamorphosed rocks of the L achlan fold belt inthe west and abutting the New England fold belt in thenortheast along the east- dipping Hunter thrust.The Sydneybasin is separated from the Gunnedah basin by the MountCoricudgy an…     

砂宝斯金矿区多矿物脉特征及其对砂岩的改造

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