Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China

To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin,the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections.According to detailed analysis of petrologic characteristics,the reservoir rock types include micritic dolomite,grain dolomite and microbialite which have mainly moldic pore,intergranular(dissolved)pore,and(dissolved)residual framework pore as main reservoir space respectively.The study area developed upward-shallowing sequences,with an exposure surface at the top of a single upward-shallowing sequence.The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling,indicating these reservoirs were related to the facies-controlled eogenetic karstification.With the increase of karstification intensity,the reservoirs became worse in physical properties.

Control factors of reservoir oil-bearing difference of Cretaceous Mishrif Formation in the H oilfield, Iraq

To reveal the law of oiliness of the Upper Cretaceous carbonate reservoirs in the Middle East and factors controlling the oiliness, the Mishrif Formation of the H oilfield in Iraq was examined comprehensively. Based on core observation and description, casting thin section analysis, and statistics of physical property and pore-throat structure parameter, etc., in combination with previous achievements and understanding, it is pointed out that the coring interval of Mishrif Formation has strong oiliness heterogeneity, and can be divided into 4 grades, oil rich, oil immersed, oil spot, and oil trace; the oil-bearing grade has obvious facies-controlled characteristic in macroscopic view; and in different oil-bearing grades, relatively oil-rich sections present higher karst development intensity, better physical properties and pore-throat structure characteristics. It is concluded through the study that the sedimentary microenvironment and eogenetic karstification determine the macroscopic oiliness of the reservoir in Mishrif Formation of H Oilfield, and the improvement of pore-throat structure caused by eogenetic karstification is the decisive factor of the micro oil-bearing difference of the reservoir. Finally,the genetic model of reservoir development and oil-bearing difference was established.

Origin of the penecontemporaneous sucrosic dolomite in the Permian Qixia Formation, northwestern Sichuan Basin, SW China

The characteristics,formation time,and origin of the sucrosic dolomite reservoirs in the Permian Qixia Formation of northwestern Sichuan Basin are analyzed.Core and outcrop description and microscopic analysis of the sucrosic dolomite samples are carried out.It is found that the dolomite has typical features different from other kinds of dolomites:(1)This dolomite is generally medium-coarse in crystal size,and often associated with very finely to finely crystalline dolomite and cave-filling dolomite.(2)Typical identification marks of eogenetic karstification are developed at the top of the upward-shallowing sequence.(3)The medium-coarse crystalline sucrosic dolomite is cut by the early diagenetic karst fabric,and is characterized by dolomite with dissolution edge,dolomite vadose silt in pores,and transgression clay filling between the medium-coarse dolomite crystals.The medium-coarse crystalline sucrosic dolomite was formed earlier than the eogenetic karstification.The sucrosic dolomite with occasional cloudy core and clear rim has bright cathodoluminescence,high inclusions temperature,significant negative skewness carbon and oxygen isotopic compositions,and rare-earth element(REE)pattern similar to seawater,indicating it experienced two periods of dolomitization,evaporative concentration reflux-infiltration and penecontemporaneous seawater circulation hydrothermal fluid dolomitization.The study results not only update the understanding on the dolomitization time of Qixia Formation,demonstrate that the sucrosic dolomite can be formed in the penecontemporaneous stage when seawater reflux superimposed with hydrothermal fluid effects,but also show that the taphorogenesis in the Dongwu period began in the Early Permian.Moreover,the dolomite controlled by the grain bank migration and terrain in the slope break appears in bands of large scale,this knowledge provides basis for expanding the exploration field of this type of reservoirs.