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Anomalously high-amplitude seismic reflections are commonly observed in deeply buried Ordovician carbonate strata in the Halahatang area of the northern Tarim Basin. These bright spots have been demonstrated to be generally related to effective oil and gas reservoirs. These bright spot reflections have complex geological origins, because they are deeply buried and have been altered by multi-phase tectonic movement and karstification. Currently, there is no effective geological model for these bright spots to guide hydrocarbon exploration and development. Using core, well logs, and seismic data, the geological origins of bright spot are classified into three types, controlled by karstification, faulting, and volcanic hydrothermal activity. Bright spots differing by geological origin exhibit large differences in seismic reflection character, such as reflection amplitude, curvature, degree of distortion, and the number of vertically stacked bright spots in the seismic section. By categorizing the bright spots and the seismic character of the surrounding strata, their geological origins can after be inferred. Reservoirs formed by early karstification were later altered by epigenetic karstification. Two periods of paleodrainage further altered the early dissolution pores. In addition, faults formed by tectonic uplift also enhanced the dissolution of the flowing karst waters. Some reservoirs were subsequently altered by Permian volcanic hydrothermal fluids.Des reflecteurs sismiques de haute amplitude sont couramment observes dans des strates carbonatees ordoviciennes profondement enfouies dans la region de Halahatang de la partie nord du bassin du Tarim. Il a ete demontre que ces reflecteurs saillants sont generalement associes a des reservoirs de petrole et de gaz. Ces reflecteurs ont des origines geologiques complexes puisqu'ils sont enfouis profondement et ont ete modifies par plusieurs phases de mouvement tectonique et de karstification. Il n'existe actuellement aucun modele geologique permettant d'utiliser ces reflecteurs pour orienter l'exploration et la mise en valeur d'hydrocarbures. Les origines geologiques de reflecteurs sont regroupees, a la lumiere de carottes, de diagraphies et de donnees sismiques, en trois types controles par la karstification, la formation de failles et l'activite hydrothermale volcanique. Les reflecteurs aux origines geologiques distinctes presentent d'importantes differences sur le plan de la sismique reflexion, comme leur amplitude, leur courbure, leur degre de distorsion et le nombre de reflecteurs empiles verticalement dans la coupe sismique. En categorisant les reflecteurs et le caractere sismique des strates qui les entourent, il est possible d'inferer leurs origines geologiques. Des reservoirs formes par une karstification precoce ont ensuite ete alteres par une karstification epige-netique. Deux periodes de paleo-ecoulement ont a nouveau modifie les pores formes par dissolution precoce. Des failles formees par le soulevement tectonique ont en outre accru la dissolution par les eaux karstiques en mouvement. Certains reservoirs ont par la suite ete alteres par des fluides hydrothermaux volcaniques permiens. [Traduit par la Redaction], IntroductionBead-like 'bright spot' reflections often appear on seismic sections across carbonate fracture-cave reservoirs, as a result of significant differences in acoustic impedance between the reservoir and the surrounding rocks. They [...]

A mature understanding of the sandstone gas reservoir in the Xiaoheba Formation in the southeastern Sichuan Basin remains lacking. To assess the reservoir characteristics and the origin of the high-quality reservoir in the Xiaoheba Formation, this paper uses systematic field investigations, physical property analysis, thin section identification, scanning electron microscopy and electron microprobe methods. The results indicate that the Xiaoheba sandstone is an ultra-tight and ultra-low permeability reservoir, with an average porosity of 2.97% and an average permeability of 0.56×10−3 μm2. This promising reservoir is mainly distributed in the Lengshuixi and Shuangliuba regions and the latter has a relatively high-quality reservoir with an average porosity of 5.28% and average permeability of 0.53×10−3 μm2. The reservoir space comprises secondary intergranular dissolved pores, moldic pores and fractures. Microfacies, feldspar dissolution and fracture connectivity control the quality of this reservoir. The relatively weak compaction and cementation in the interbedded delta front distal bar and interdistributary bay microfacies indirectly protected the primary intergranular pores and enhanced late-stage dissolution. Late-stage potassium feldspar dissolution was controlled by the early-stage organic acid dissolution intensity and the distance from the hydrocarbon generation center. Early-stage fractures acted as pathways for organic acid migration and were therefore important factors in the formation of the reservoir. Based on these observations, the area to the west of the Shuangliuba and Lengshuixi regions has potential for gas exploration. [ABSTRACT FROM AUTHOR]