Sequence structure and non-structural traps of the Paleogene in the Weixi'nan Sag, Beibuwan Basin

Based on Vail sequence stratigraphic theory and analysis of seismic and logging information, the sequence in the Paleogene Liushagang and Weizhou formations in the Weixi'nan Sag is classified, the strata framework of sequence is defined and established, the characteristics of sequence structure are analyzed and the distribution of non-structural traps is revealed. One first-order sequence boundary, two second-order sequence boundaries, eight third-order sequence boundaries and ten third-order sequences are identified in the Liushagang and Weizhou Formations. According to the characteristics of sequence boundary structure, the third-order sequence boundaries can be divided into three types: type T1, type T2, and type T3. And according to the characteristics of sequence stratigraphic structure, the ten third-order sequences can be classified into four types: one-divided sequence, two-divided sequence, three-divided sequence, and four-divided sequence. The favorable areas for the development of stratigraphic overlap traps are above type T1 and type T3 sequence boundaries, while the unconformity barrier traps are developed below type T2 and type T3 sequence boundaries. The favorable development periods of lithologic traps are the early period of three-divided sequence and four-divided sequence and the late period of two-divided sequence and four-divided sequence. Key words: sequence stratigraphy, sequence boundary, sequence structure, non-structural trap, Weixi'nan Sa

Factors influencing oil saturation and exploration fairways in the lower cretaceous Quantou Formation tight sandstones, Southern Songliao Basin, China

Favorable exploration fairway prediction becomes crucial for efficient exploration and development of tight sandstone oil plays due to their relatively poor reservoir quality and strong heterogeneous oil saturation. In order to better understand the factors influencing oil saturation and favorable exploration fairway distribution, petrographic investigation, reservoir properties testing, X-ray diffraction analysis, oil saturation measurement, pressure-controlled mercury injection, and rate-controlled mercury injection were performed on a suite of tight reservoir from the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4 ) in the southern Songliao Basin, China. The sandstone reservoirs are characterized by poor reservoir properties and low oil saturations. Reservoir properties between laboratory pressure conditions and in situ conditions are approximately the same, and oil saturations are not controlled by porosity and permeability obviously. Pores are mainly micro-scale, and throats are mainly nano-scale, forming micro- to nano-scale pore–throat system with effective connected pore–throat mainly less than 40%. Oil emplacement mainly occurs through the throats with average radius larger than 0.25 µm under original geological condition. Moreover, the samples with higher oil saturation show more scattered pore and throat distributions, but centered pore–throat radius ratio distribution. Pore–throat volume ratio about 2.3–3.0 is best for oil emplacement, forming high oil saturation. Quartz overgrowth, carbonate cements, and authigenic clays are the major diagenetic minerals. The reservoirs containing about 4–5% carbonate cements are most preferable for oil accumulation, and oil saturation increases with increasing of chlorite as well. The flow zone indicator is a reasonable parameter to predict favorable exploration targets in tight sandstone reservoirs. The reservoirs with flow zone indicator values larger than 0.05 can be regarded as favorable exploration targets in the K 1 q 4 tight sandstones. According to the planar isoline of average flow zone indicator value, the favorable exploration targets mainly distribute in the delta plain distributary channel and deltaic front subaqueous distributary channel

Diagenesis and reservoir quality of sandstones with ancient “deep” incursion of meteoric freshwater——An example in the Nanpu Sag, Bohai Bay Basin, East China

An example of diagenesis and reservoir quality of buried sandstones with ancient incursion of meteoric freshwater is presented in this study. The interpretation is based on information including porosity and permeability, petrography, stable isotopic composition of authigenic minerals, homogenization temperatures (Th) of aqueous fluid inclusions (AFIs), and pore water chemistry. These sandstones, closely beneath or far from the regional unconformity formed during the late Paleogene period, are located in the thick Shahejie Formation in the Gaoliu area of Nanpu Sag, Bohai Bay Basin, East China. Early-diagenetic calcite cements were leached to form intergranular secondary pores without precipitation of late-diagenetic calcite cements in most sandstones. Feldspars were leached to form abundant intragranular secondary pores, but with small amounts of concomitant secondary minerals including authigenic quartz and kaolinite. The mass imbalance between the amount of leached minerals and associated secondary minerals suggests that mineral leaching reactions occurred most likely in an open geochemical system, and diagenetic petrography textures suggest that advective flow dominated the transfer of solutes from leached feldspars and calcites. Low salinity and ion concentrations of present pore waters, and extensive water rock interactions suggest significant incursion of meteoric freshwater flux in the sandstones. Distances of the sandstones to the regional unconformity can reach up to 1800 m, while with significant uplift in the Gaoliu area, the burial depth of such sandstones (below sea level) can be less than 800–1000 m during the uplift and initial reburial stage. Significant uplift during the Oligocene period provided substantial hydraulic drive and widely developed faults served as favorable conduits for downward penetration of meteoric freshwater from the earth's surface (unconformity) to these sandstone beds. Extensive feldspar leaching has been occurring since the uplift period. Coupled high Th (95∼115 °C) of AFI and low δ18O(SMOW) values (+17∼+20‰) within the quartz overgrowths show that quartz cementation occurred in the presence of diagenetic modified meteoric freshwater with δ18O(SMOW) values of −7∼−2‰, indicating that authigenic quartz only have been formed during the late reburial stage when meteoric fresh water penetration slowed down. Secondary pores in thin sections and tested porosity suggest that meteoric freshwater leaching of feldspars and calcite minerals generated approximately 7–10% enhanced secondary porosity in these sandstones. Meteoric freshwater leaching reactions cannot be ignored in similar sandstones that located deep beneath the unconformity, with great uplift moving these sandstones above or close to sea level and with faults connecting the earth's surface with the sandstone bed

Reactive transport modeling of coupled feldspar dissolution and secondary mineral precipitation and its implication for diagenetic interaction in sandstones

Dissolution of feldspars and precipitation of secondary minerals (kaolinite, illite and quartz) are significant diagenetic processes in arkosic sandstones. We examined moderately buried sandstones in the Eocene Shahejie Formation from two sags in the Bohai Bay Basin, East China. Three different types of mineral assemblages (MA) were identified: extensively leached feldspars with a large amount of authigenic kaolinite and quartz cement (MA-1), extensively leached feldspars with a large amount of authigenic kaolinite and minor quartz cement (MA-2), and extensively leached feldspars with a small amount of both authigenic kaolinite and quartz cement (MA-3). Numerical simulations at the continuum scale using Geochemist’s Workbench 9.0 were conducted to decipher the origin of the different mineral assemblages. The physicochemical reactions including feldspar dissolution, transport of Al3+ and SiO2(aq), and precipitation of kaolinite and quartz are coupled together in these simulations, with constraints of chemical reactions, kinetic law, dispersion, and advection. Modeling results suggest that a dissolution zone, a transitional zone, and a precipitation zone can be formed in a sandstone unit with suitable constraints of temperature, flow rate, fluid composition and mineral reaction rate. And MA-3, MA-2, and MA-1 assemblages develop in these three zones respectively. The higher SiO2(aq) concentration required for the saturation of quartz than for kaolinite and the low Al3+ concentration needed for the saturation of kaolinite lead to the precipitation of only kaolinite in the transitional zone in a geochemical system with feldspar dissolution serving as the dominant source of SiO2(aq) and Al3+. Comparisons between modeling results and observations of natural sandstone diagenesis suggest that an MA-1 assemblage is likely to occur in buried sandstones at high temperatures (>70–80 °C) and low flow rates. An MA-2 assemblage may occur in moderately buried sandstones at moderate temperatures (40–70 °C), in deeply buried sandstones with faults and fractures serving as conduits of meteoric freshwater, or in shallow sandstones where meteoric water is not abundant. An MA-3 assemblage tends to occur in shallow sandstones at low temperatures (<40–50 °C) and high flow rates, or in buried sandstones where faults and fractures develop widely and serve as freshwater conduits. These proposals are valid in natural arkosic sandstones and of great significance in deciphering the diagenetic environments where the feldspar dissolution and secondary mineral precipitation have occurred

Genesis of granular calcite in lacustrine fine-grained sedimentary rocks and its indication to volcanic-hydrothermal events: A case study of Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, NW China

Granular calcite is an authigenic mineral in fine-grained sedimentary rocks. Core observation, thin section observation, cathodoluminescence analysis, fluid inclusion analysis, scanning electron microscope (SEM), and isotopic composition analysis were combined to clarify the genesis of granular calcite in the lacustrine fine-grained sedimentary rocks of the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin. It is found that the granular calcite is distributed with laminated characteristics in fine-grained sedimentary rocks in tuffite zones (or the transitional zone between tuffite and micritic dolomite). Granular calcite has obvious cathodoluminesence band, and it can be divided into three stages. Stage-I calcite, with non-luminesence, high content of Sr element, inclusions containing COS, and homogenization temperature higher than 170 °C, was directly formed from the volcanic-hydrothermal deposition. Stage-II calcite, with bright yellow luminescence, high contents of Fe, Mn and Mg, enrichment of light rare earth elements (LREEs), and high homogenization temperature, was formed by recrystallization of calcareous edges from exhalative hydrothermal deposition. Stage-III calcite, with dark orange luminescence band, high contents of Mg, P, V and other elements, no obvious fractionation among LREEs, and low homogenization temperature, was originated from diagenetic transformation during burial. The granular calcite appears regularly in the vertical direction and its formation temperature decreases from the center to the margin of particles, providing direct evidences for volcanic-hydrothermal events during the deposition of the Lucaogou Formation. The volcanic-hydrothermal event was conducive to the enrichment of organic matters in fine-grained sedimentary rocks of the Lucaogrou Formation, and positive to the development of high-quality source rocks. The volcanic-hydrothermal sediments might generate intergranular pores/fractures during the evolution, creating conditions for the self-generation and self-storage of shale oil

The timescale of plume-driven cratonization: A complete record from Tarim

ABSTRACT: Cratonization of the Tarim block in central Asia is finalized by the Permian Tarim plume that welded two cratonic nuclei together. Hence, the over-10-km-thick Tarim basin preserves a complete record of deformation and growth strata before, during, and after the plume-driven cratonization. Here we use seismic reflection data from the central Tarim basin to quantify the timing and style of the Paleozoic–Mesozoic deformation. The thrust and strike-slip faults there all underwent an early, intense deformation stage in the earliest Ordovician–Middle Devonian, a hiatus stage from Late Devonian to Late Permian, and a newly-discovered stage of weak activity throughout the Mesozoic. The intracontinental deformation is controlled by the subduction and accretion surrounding the Tarim block. The minor, but non-zero, Mesozoic strains reflect the ongoing adjustment to far-field compressions during the cooling and strengthening of the plume-stitched continental lithosphere. The cessation of interior deformation marks that the Tarim cratonization is finally attained ~200 Myr after the plume waned

Geologic CO2 storage in arkosic sandstones with CaCl2-rich formation water

The feasibility of geologic CO2 storage in deeply buried arkosic sandstones has been tested using high-temperature, high-pressure short-term laboratory experiments and long-term numerical simulations with CO2-saturated solution rich in CaCl2. These conditions mimic the conditions found today in the Eocene reservoir sandstones of depleted oilfields in the Dongying Sag, Bohai Bay Basin, China. Experiments at 100 °C and 150 °C and PCO2 of 4 MPa were conducted on sandstones rich in K-feldspar and albite without anorthite. During the experiments, calcite and kaolinite precipitated while albite and K-feldspar partly dissolved. Ca2+ in formation water is shown to be critical for mineral trapping of CO2. The continuous dissolution of K-feldspar and albite at a slow rate for a long time period can prolong duration time of calcite precipitation and increase geologic CO2 storage capability by mineral trapping. Addition of NaCl, KCl and MgCl2 can prolong the dissolution time of K-feldspar and albite and precipitation duration of calcite. It also increased the mass of sequestered CO2 by mineral trapping. The process of geologic CO2 storage can be divided into 3 stages. In stage I, calcite precipitates rapidly, and geologic CO2 storage is dominated by solubility trapping within 100 years. Stage II lasts up to 300 years in the solution without NaCl, KCl and MgCl2 and 900 years with addition of NaCl, KCl and MgCl2 into the solution. The precipitation rate of calcite decreases abruptly and the mass of mineral trapping of CO2 increases with increasing time, because dissolution of K-feldspar and albite causes decrease of Ca activity and prolongs calcite precipitation time. The geologic CO2 storage is dominated by mineral trapping. In stage III, reactions reach equilibrium and the mass of geologic CO2 storage reaches the maximum. The calculated mass of CO2 by mineral trapping in sandstones of the Es4x to Es3z in the Dongying Sag is about 3.61Gt