Burial Dissolution of Ordovician Granule Limestone in the Tahe Oilfield of the Tarim Basin,NW China,and Its Geological Significance

With a comprehensive study on the petrology, geology and geochemistry of some Ordovician granule limestone samples in the Tahe Oiifieid of the Tarim Basin, two stages of burial dissolution were put forward as an in-source dissolution and out-source dissolution based on macro-microcosmic petrology and geochemistry features. The main differences in the two stages are in the origin and moving pass of acid fluids. Geochemical evidence indicates that burial dissolution fluids might be ingredients of organic acids, CO2 and H2S associated with organic matter maturation and hydrocarbon decomposition, and the in-source fluid came from organic matter in the granule limestone itself, but the out-source was mainly from other argillaceous carbonate rocks far away. So, the forming of a burial dissolution reservoir resulted from both in-source and the out-source dissolutions. The granule limestone firstly formed unattached pinholes under in-source dissolution in situ, and afterwards suffered wider dissolution with out-source fluids moving along unconformities, seams, faults and associate fissures. The second stage was much more important, and the mineral composition in the stratum and heat convection of the fluid were also important in forming favorable reservoirs.

The Role of Seal and Source Rocks in Fluid-rock Interactions and their Reservoir Effects within the Carbonate Deep Burial Realm

Discoveries of deep high-quality carbonate reservoirs challenged the general understanding on the evolution of porosity decreasing with depth.New mechanisms of pore generation and preservation in the deep realm require to be proposed.Dolostones in the Feixianguan and Dengying Formations experienced maximum depths in excess of 8000 m,but still retained high porosity.Petrographic observation and homogenization temperatures help to identify products of deep fluid-rock interactions,visual and experimental porosity were used to quantify reservoir effects,the distribution of products finally being plotted to unravel the mechanisms.Th data reveal that thermochemical sulfate reduction(TSR),burial dissolution and quartz cementation are typical deep fluid-rock interactions.The SO_(4)^(2-)of residual porewater sourced from the evaporative dolomitizing fluid was supplied for TSR in the hydrocarbon column,the TSR-inducing calcite cements were homogeneously dispersed in the hydrocarbon column.Quartz cementation was caused by the increasing acidity and Si-rich residual porewater in the oil column.Burial dissolution is forced by organic acid and limited in oil-water contact.This study suggests that seal and source rocks not only play important roles in hydrocarbon accumulation,but also have a general control on the deep fluid-rock interactions and porosity evolution in the deep burial realm.

Primary Intergranular Pores in Oolitic Shoal Reservoir of Lower Triassic Feixianguan Formation,Sichuan Basin,Southwest China:Fundamental for Reservoir Formation and Retention Diagenesis

The Lower Triassic Feixianguan （飞仙关） Formation oolitic shoal reservoir in the Sichuan （四川） basin （Southwest China） is currently an exploration and research highlight in China. The reservoir is widely believed to be formed mainly by burial dissolution and/or dolomitization on the basis of primary intergranular pores. In this study, through a comprehensive geological study on the whole basin, the dissolution and dolomitization are suggested not to be the fundamental factor of reservoir formation and there thus may be a possible new fundamental mechanism-the preservation of primary intergranular pores, i.e., the retention diagenesis. Based on this, a complex and multi-stage reservoir evolution and formation model is proposed. In the model, the depositional environment is the basis of reservoir initial formation. Subsequently, early compaction and shallow burial cementation result in the primary reservoir differentiation. Then, multi-stage burial dissolution alters and adjusts the reservoir. Because the last stage gaseous hydrocarbons have little diagenetic impact, the reservoir is formed finally. Therefore, this study presents a possible new fundamental mechanism and evolution model for the reservoir formation. The results can be applied in the regional reservoir predication and shaping exploration strategies, and provide reference for the study of shoal reservoirs in other areas.