Tight Sandstone Reservoir Formation Mechanism of Upper Paleozoic Buried Hills in the Huanghua Depression, Bohai Bay Basin, Eastern China

Carboniferous-Permian petroleum resources in the Huanghua Depression of the Bohai Bay Basin, a super petroleum basin, are important exploration successor targets. The reservoir sedimentary environment of coal measures in the Upper Paleozoic buried hills is variable, and the structural evolution process is complicated, which restricts the optimization of targeting sections. Using the analysis and testing results of logging, thin section, porosity, mercury injection, hydrochemistry, and basin simulation, this study revealed the formation mechanism differences of tight sandstones in the Upper Paleozoic period in different buried hills. The results show that the sandstones are mainly feldspathic sandstone, lithic arkose, feldspathic lithic sandstone, and feldspathic lithic quartz sandstone. The quartz content varies between 25% and 70%, averaging 41%. Feldspar and debris are generally high, averaging 31% and 28%, respectively. Secondary dissolution pores are the main reservoir spaces, with 45% of the tested samples showing porosity of 5–10%, and 15% being lower than 5%. The pore radium is generally lower than 100 nm, and the sandstones are determined as small pore with fine throat and medium pore with fine throat sandstones by mercury saturation results. Frequent changing sedimentary environments and complex diagenetic transformation processes both contribute to the reservoir property differences. The former determines the original pore space, and the latter determines whether they can be used as effective reservoirs by controlling the diagenetic sequences. Combining tectonic movement background and different fluid history, the different formation mechanisms of high-porosity reservoirs are recognized, which are atmospheric leaching dominated (Koucun buried hills), atmospheric water and organic acid co-controlled (Wangguantun and Wumaying buried hills), and organic acid dominated (Nandagang buried hills) influences. The results can be beneficial for tight gas exploration and development in coal measures inside clastic buried hills in the Bohai Bay Basin.

Structure and Petrology of the Red Hill Complex, Nelson

<p>The Red Hill Complex is an essentially concordant ultramafic body enclosed in Upper Paleozoic flysch facies sediments which include Pelorus Group (oldest), Lee River Group and Maitai Group. The Pelorus Group contains rare submarine lavas and is largely derived from spilitic volcanics. The Lee River Group consists of spilitic pillow lavas, volcanic breccias and spilitic basalts and dolerites. The Maitai Group consists of limestone, sandstone and argillite; an extensive conglomerate lens in the argillites is largely composed of andesitic pebbles. The Red Hill Complex is a 12,000 ft. thick lens and is part of a sheet of peridotites which may extend 40 miles northward to Dun Mountain. The Complex is divided into a 3000 ft thick Basal Zone of massive harzburgite and a 9000 ft thick Upper Zone of layered harzburgite and dunite with minor variants, feldspathic-peridotite, eucrite, lherzolite, wehrlite and pyroxenite. The bulk composition of both zones is approximately the same but the Upper Zone contains about 0.2 per cent feldspar not present in the Basal Zone. There is no significant regional change in mineral chemistry throughout the Complex and the average composition is about; olivine Fo91, 70 per cent; orthopyroxene, En88, 22 per cent; clinopyroxene, 5 per cent; feldspar An96, less than 0.2 per cent; spinel 2 per cent. Layering and foliation are common in the top of the Upper Zone. Layering is of at least two generations of which at least one is of metamorphic origin. Metamorphic layering was formed by metasomatic replacement probably along subhorizontal shear planes during intrusion of the ultramafic sheet. Pyroxene pegmatites formed after flow ceased. The diversity of rock types in the top of the Upper Zone is considered by the writer to have been caused by metamorphic differentiation of parent material the same composition as the Basal Zone. The preferred orientation of olivine in lineated, foliated, laminated and layered rocks has the same pattern suggesting a close genetic relationship between those structures. Evidence strongly supports a tectonic origin for the preferred orientation. Rocks in the Upper Zone are xenomorphic-granular in texture and those in the Basal Zone are typically protoclastic. Xenomorphic-granular textures are derived in part from protoclastic by post-deformational recrystallization. The ultramafic rocks are cut by a number of dykes composed of hornblende-labradorite, hypersthene-augite-bytownite assemblages or minor variants of these. The dykes were intruded shortly after emplacement of the ultramafic rocks. The Red Hill Complex is considered to have been emplaced as a sheet at shallow depths which intruded superficial deposits on the ocean floor and was later overlain by volcanics</p>

Structure and Petrology of the Red Hill Complex, Nelson

<p>The Red Hill Complex is an essentially concordant ultramafic body enclosed in Upper Paleozoic flysch facies sediments which include Pelorus Group (oldest), Lee River Group and Maitai Group. The Pelorus Group contains rare submarine lavas and is largely derived from spilitic volcanics. The Lee River Group consists of spilitic pillow lavas, volcanic breccias and spilitic basalts and dolerites. The Maitai Group consists of limestone, sandstone and argillite; an extensive conglomerate lens in the argillites is largely composed of andesitic pebbles. The Red Hill Complex is a 12,000 ft. thick lens and is part of a sheet of peridotites which may extend 40 miles northward to Dun Mountain. The Complex is divided into a 3000 ft thick Basal Zone of massive harzburgite and a 9000 ft thick Upper Zone of layered harzburgite and dunite with minor variants, feldspathic-peridotite, eucrite, lherzolite, wehrlite and pyroxenite. The bulk composition of both zones is approximately the same but the Upper Zone contains about 0.2 per cent feldspar not present in the Basal Zone. There is no significant regional change in mineral chemistry throughout the Complex and the average composition is about; olivine Fo91, 70 per cent; orthopyroxene, En88, 22 per cent; clinopyroxene, 5 per cent; feldspar An96, less than 0.2 per cent; spinel 2 per cent. Layering and foliation are common in the top of the Upper Zone. Layering is of at least two generations of which at least one is of metamorphic origin. Metamorphic layering was formed by metasomatic replacement probably along subhorizontal shear planes during intrusion of the ultramafic sheet. Pyroxene pegmatites formed after flow ceased. The diversity of rock types in the top of the Upper Zone is considered by the writer to have been caused by metamorphic differentiation of parent material the same composition as the Basal Zone. The preferred orientation of olivine in lineated, foliated, laminated and layered rocks has the same pattern suggesting a close genetic relationship between those structures. Evidence strongly supports a tectonic origin for the preferred orientation. Rocks in the Upper Zone are xenomorphic-granular in texture and those in the Basal Zone are typically protoclastic. Xenomorphic-granular textures are derived in part from protoclastic by post-deformational recrystallization. The ultramafic rocks are cut by a number of dykes composed of hornblende-labradorite, hypersthene-augite-bytownite assemblages or minor variants of these. The dykes were intruded shortly after emplacement of the ultramafic rocks. The Red Hill Complex is considered to have been emplaced as a sheet at shallow depths which intruded superficial deposits on the ocean floor and was later overlain by volcanics</p>

Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

Upper Paleozoic reef systems of the Rubezhinsky Trough (southern part of the Buzuluk Depression)

Background. The Rubezhinsky Trough has been remained one of the most poorly studied petroleum areas of the Orenburg region to date. At present, Gazprom Neft conducts systematic exploration work within the trough at five license areas of the South Orenburg Cluster. Aim. The purpose of this article is representation of preliminary new dates on a geological structure of paleozoic reef systems within the Rubezhinsky Trough. Materials and methods. The main original materials for the work are the results of the interpretation of the 3D seismic carried out at four license areas. In addition, results drilling and regional sedimentation models were involved for analysis. Results. Regional models of Upper Paleozoic reef systems of the Rubezhinsky Trough have been extensively detailed as a result of the interpretation of 3-D seismic data within the South Orenburg cluster. It was first established that isolated reefs were formed in the interval of the Ardatovian and Mullinian regional stages (Givetian Stage of Middle Devonian) of the research area. Ardatovian-mullinian isolated reefs are covered with clay deposits and represent potential lithological traps for petroleum deposits. Isolated reefs, isolated carbonate platforms and the southern margin of the vast South-Buzuluk carbonate platform with barrier reef systems have been identified for the Frasnian Stage. Isolated frasnian reefs are potential hydrocarbon traps. Barrier frasnian reefs together with increasing them early famennian ones form a series of structural hydrocarbon traps in the overlapping complexes. The significant progradation of the margin of the famennian carbonate platform towards the Pre-Caspian paleobasin is established. Famennian progradation complexes form several large clinoforms which are potentially forward looking for the search for structural-lithological petroleum traps. The barrier reef system has been confirmed for the evaporite-carbonate complex of the Okskian regional stage. Okskian reefs border the late visean epicratonic carbonate platform. Relatively large reefs of the carbonate platform barrier system were identified in the interval from Podolskian regional stage (Carboniferous) to Asselian Stage (Permian). This barrier system has progradational architectures towards the Pre-Caspian paleobasin that was formed from the end of the Middle Carboniferous to the end of the Artinskian Age of the Early Permian. Podolskian-asselian barrier buildings predefine the development of structural hydrocarbon traps of various sizes in overlapping Lower Permian deposits. Conclusions. A preliminary analysis of 3-D seismic data indicates the significant role of the paleozoic reef systems in the formation of the sedimentary complex of the Rubezhinsky trough.