Source Area Weathering and Tectonic History Inferred from the Geochemistry of the Maastrichtian Sandstone from Patti Formation,Southern Bida Basin,North Central Nigeria

Sandstones sampled from Patti Formation,Southern Bida Basin,were studied geochemically using Inductively Coupled Plasma Atomic Emission Spectrophotometer(ICP-AES)and an Inductively Coupled Plasma Mass Spectrometry(ICP-MS)technique to evaluate their weathering and tectonic setting as well as to deduce the paleo-climatic conditions that existed during their deposition.Geochemically,SiO_(2)range from 73.9%to 86.2%,Al_(2)O_(3)(6.7%~17.1%),Fe_(2)O_(3)(1.1%~1.9%),K_(2)O(0.1%~0.7%)while MgO,CaO,Na_(2)O,P_(2)O_(5),MnO and TiO_(2)were<1%.Enriched in Ba(Av.622.94),Sr(Av.153.63),Rb(Av.55.08)and Zr(Av.51.86)relatively similar in composition to UCC.High SiO_(2)but low other major oxides signify high mobility during processes of weathering.This was confirmed by high value(>80%)for indices like chemical index of alteration,chemical index of weathering,plagioclase index of alteration,mineralogical index of alteration and relatively lower values for weathering index of parker,recently used alpha indices(α^(Al)_(E))of sodium(326.17α^(Al)_(Na)<344.40),magnesium(100.54α^(Al)_(mg)<398.55),calcium(12.07α^(Al)_(Ca)<198.99),potassium(4.43α^(Al)_(K)<64.33),strontium(0.84α^(Al)_(E)<21.40),barium(0.45α^(Al)_(Ba)<10.52)and rubidium(0.0008α^(Al)_(Rb)<0.06),supported by AI_(2)O_(3)-(CaO^(*)+Na_(2)O)-K_(2)O and CIA vs.SiO_(2)plots that imply intense weathering in the source area.The obtained high CIA values(>80)indicates a steady-state of weathering under a warm/humid climate as confirmed by the SiO_(2)vs.Al_(2)O_(3)+K_(2)O+Na_(2)O plot.High average SiO_(2)(75.41wt%)with K_(2)O/Na_(2)O ratio>1(15.63),low Fe_(2)O_(3)(1.27wt%),Al_(2)O_(3)(15.82wt%)and TiO_(2)(0.46)suggest passive margin tectonic setting.This is supported by enrichedΣREE(209.64 ppm),ΣLREE(195.78),LREE/HREE(27.78)and negative Eu/Eu^(*)(0.68),plots of log(K_(2)O/Na_(2)O)vs.SiO_(2)and SiO_(2)/Al_(2)O_(3)vs.K_(2)O/Na_(2)O.Major elements discriminant-function multi-dimensional diagram,DF1(arcrift-col)vs.DF2(arc-rift-col),for high-silica sediments revealed a continental rift tectonic setting.Thus,the Patti Formation sandstone underwent a high degree of weathering under a humid climatic condition within a continental rift tectonic setting.

An integrated laboratory experiment of realistic diagenesis,perforation and sand production using a large artificial sandstone specimen

Sand production is,a challenging issue in petroleum industry,mainly associated with weak unconsolidated formations.A novel testing procedure and a new apparatus were developed to conduct an integrated experiment of diagenesis,perforation and sand production on a single large cylindrical artificial sandstone specimen,where solid and fluid pressures can be independently controlled such that realistic reservoir historical conditions can be well simulated in the laboratory.Fluid injection can be performed in both radial and vertical directions,where both single-and two-phase flows can be implemented for study of sand production behaviors at different reservoir’s maturity stages.The equipment consists of an intensive instrumentation system to monitor pressures,displacements and material states continuously.The produced sand particles were filtered and monitored in real-time for the study of time-dependent phenomena.The experimental results showed similar patterns to that observed in the field and provided valuable insight for the development of prediction methods for sand production of similar materials.

Influence of strong preformed particle gels on low permeable formations in mature reservoirs

In mature reservoirs,the success of preformed particle gel（PPG） treatment rests primarily on the ability of the PPG to reduce and/or plug the high permeable formations,but not damage the low permeable formations.Static test models（filtration test model and pressure test model）were used to determine the effect of PPG on low permeable formations.This work used a strong preformed particle gel,Daqing（DQ） gel made by a Chinese company.The particle gel sizes were ranged from 30 to 120 mesh for this work.PPGs are sized in a millimeter or micrometer,which can absorb over a hundred times their weight in liquids.The gel strength was approximately 6500 Pa for a completely swollen PPG with 1 %（weight percentage） NaCl solution（brine）.0.05 %,1 %,and 10 % NaCl solutions were used in experiments.Sandstone core permeability was measured before and after PPG treatments.The relationship between cumulative filtration volumes versus filtration times was determined.The results indicate that DQ gels of a particle size of 30–80 mesh did not damage the cores of a low permeability of 3–25 m D.The DQ gels of a smaller particle size ranging from 100 to 120 mesh damaged the core and a cake was formed on the core surface.The results also indicate that more damage occurred when a high load pressure（400 psi） was applied on the high permeability cores（290–310 m D）.The penetration of the particle gelsinto the low permeable formations can be decreased by the best selection of gel types,particle sizes,and brine concentrations.

Architecture of deepwater turbidite lobes: A case study of Carboniferous turbidite outcrop in the Clare Basin, Ireland

Through lithofacies analysis and architecture anatomy of the Carboniferous Ross Sandstone turbidites outcropped at western Ireland,the depositional model of deepwater turbidite lobes is established.Seven types of lithofacies are recognized including goniatites-rich shale,laminated shale,laminated siltstone,massive sandstone,fine-medium sandstone with mud-clast,basal gravel,and chaotic mudstone,which can be subdivided into units of three origins,turbidite lobe,turbidite channel,and slide-slump;and four hierarchical levels,lobe complex,lobe,lobe element and single sandstone layer.The lobes show apparent compensational stacking pattern,lobe elements display typical thickening-upward cycles on vertical profile,and the higher the hierarchical level,the better the preservation of the hierarchical boundary is.In general,turbidite lobe deposits appear as tabular,parallel/sub-parallel sandstone and mudstone interbeds,and change from thick,massive sandstone in the proximal end to thinner sandstone and mudstone interbeds from axis to fringe,with the sand-shale ratio and degree of sandstone amalgamation decreasing.

Fractal characteristic of microscopic pore structure of tight sandstone reservoirs in Kalpintag Formation in Shuntuoguole area,Tarim Basin

Using the fractal geometry method,the microscopic pore structures of tight sandstone reservoirs in Kalpintag Formation of Shuntuoguole area in Tarim Basin were conducted fractal characterization on the base of test analysis data such as physical property,cast thin section,scanning electron microscope and mercury injection,and the genetic mechanism of pore structure heterogeneity was investigated.The storage spaces are dominated by intergranular dissolved pore,intragranular dissolved pore and residual intergranular pore,and the throat type consists of the necking throat,lamellar throat,curved lamellar throat and tube-shaped throat.The microscopic structure type includes Type Ⅰ(fractal dimension≤2.350),Type Ⅱ(2.350<fractal dimension<2.580),Type Ⅲ(fractal dimension>2.580)and fracture type.The most favorable reservoirs with Type-Ⅰ microscopic pore structure are mainly distributed in the Upper Member of Kalpintag Formation,while the reservoirs with Type-Ⅱ and Type-Ⅲ microscopic pore structures are mainly in the Lower Member of Kalpintag Formation.The sedimentation controls the heterogeneity of microscopic pore structure,and the differences on composition and particle size of sandstone lead to differentiation of microscopic pore structures.The Lower Member of the Kalpintag Formation experiences stronger compaction and cementation but weaker dissolution than the Upper Member of the Kalpingtag Formation,and thus the microscopic pore structure of Upper Member of the Kalpintag Formation is significantly worse that of the Lower Member o the Kalpingtag Formation.The Upper Member of the Kalpintag Formation with high content of brittle mineral develops microscopic fractures due to tectonic rupture,thus the permeability is improved and the heterogeneity of microscopic pore structures is enhanced;but the Lower Member of Kalpintag Formation is characterized by attrition crushing of particles and strong compaction.

Siliciclastic-carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone(Ariyalur, India)

Mixed siliciclastic-carbonate rocks constitute the Upper Cretaceous Garuda- mangalam Sandstone Formation, Ariyalur （India）, and offer an opportunity to look into the broad spectrum of mixing of compositionally and genetically different components. The palaeogeographic reconstruction indicates that deposition in the nearshore zone differed strongly in energy and active processes operatives due to the presence of a shore-parallel river-mouth bar. The western wing of the Mississippi bird-foot delta is considered to be a present-day analogon. Facies analysis in combination with petrography clearly shows the variability in palaeoenvironmental characteristics, both biogenic and non-biogenic. It also indicates dia- genetic uptake of carbonate that filled empty spaces and actively replaced original compo- nents. Chemical staining followed by limited application of cathodoluminescence and energy dispersive X-ray analysis （EDAX） hint at intricacies in mixing arising from the compositional variations in the carbonate components. A model of siliciclastic-carbonate sediment mixing, including both the depositional and diagenetic developments, is presented; it is aimed at generating a better overview of, and a deeper insight into, the physical and chemical mechanisms involved.

Palaeogeographic reconstruction of a fluvio-marine transitional system in Narmada rift basin, India——Implications on Late Cretaceous global sea-level rise

Rising sea-levels in tectonically active epicontinental basins often lead to varied depositional settings and palaeogeography, mostly influenced by the net accommodation resulting from mutual interference of the extent and nature of landward encroachment by the sea and the net sedimentation. The Cenomanian Nimar Sandstone Formation, Bagh Group, Narmada rift basin, uniquely portrays the effect of sea-level rise within an intra-cratonic setting and attributes to the corresponding palaeogeographic changes in west-central India. An integrated sedimentological–sequence-stratigraphic study of the broadly fining-upward Nimar Sandstone Formation(thickness~ 20–30 m) depicts the actual nature of changeover from a fluvial to a marine-dominated transitional depositional setting. Detailed sedimentological study reveals total seventeen facies, grouped in five facies associations, viz., the channel-fill facies association(FA-1), the overbank facies association(FA-2), the fluvial-dominated fluvio-tidal facies association(FA-3), the tide-dominated fluvio-tidal facies association(FA-4), and the shoreface facies association(FA-5). Overall facies architecture indicates a west-to-eastward marine encroachment, resulting in stacking of three distinct palaeo-depositional conditions:(i) an initial fluvial system with channel and overbank, changing into a tideinfluenced fluvial bay-head delta in the inner estuary, followed by(ii) marine encroachment leading to a tidedominated central estuary with inter-to sub-tidal settings, and finally,(iii) with further intense marine encroachments, a wave-reworked open shore condition in the outer estuary zone. The overall fining-up succession with a systematic change from fluvial to marine-dominated depositional systems points to a landward shift of the shoreline, signifying a major transgressive event correlated to the Cenomanian global sea-level rise. Characteristic stratal stacking patterns point to four coarsening-and fining-up hemicycles, embedded within the major transgressive succession. These high-frequency cycles attest to the varied interplay of sedimentation, tectonics and sea-level changes, and the resultant net accommodations. A palaeogeographic model is proposed based on the high-frequency transgressive–regressive hemicycles, which envisages the evolution of the depositional environments in relation to the Cenomanian eustatic rise in the intra-cratonic riftogenic fluvio-marine transitional basinal setup.