The Effects of Acid Fluid on Diagenesis in Coal Measure Strata

Abstract

Abstract

Based on thin section observation and homogenization temperature of the fluid inclusions analysis, combining with reservoir modeling, the reservoir characteristics of Jurassic coal measure strata and neighboring sandstone and the evolution of pores in Lenghu Tectonic Zone of the North Margin of the Qaidam Basin has been studied. The analysis results indicated dissolution by organic acid fluid, together with sequence boundary and favorable sedimentary facies, is one of the main controlling factors on reservoir quality.

In early diagenesis stage, the decaying plant decomposed quickly and then produced humic acid, resulting in acid medium of pore water in the process of sedimentation. Therefore, in early diagenesis most reservoir of coal measure strata lacks calcite, gypsum and zeolite cement filling, etc. And the compaction of deposit caused primary porosity decreasing, throat thinning and permeability reducing. The coal measure strata in this stage are commonly low porosity and permeability reservoir. In middle diagenesis stage, a large amount of organic acid fluid formed and intensively dissolved the unstable components in rock such as feldspar, debris and etc. As the rock in this stage has almost been consolidated and the compaction rate decreased, the secondary dissolution pore was apt to preserve and thus relatively high quality reservoir formed.

In addition, the acid fluid from coal bearing strata went into the neighboring reservoir along the fault in Lenghu Tectonic Zone. By reservoir modeling, the effects of acid fluid on porosity of reservoir were simulated. As the acid fluid migration distance increased, the quality of reservoir got better and then deteriorated before ameliorating. The diagenesis of acid fluid in different condition can be modeled by changing the related parameters. Owing to the acid fluid in Jurassic coal measure strata, the reservoir of Jurassic, neighboring Lulehe and Lower Ganchaigou Group is of high quality, despite the upper Lulehe Group. The experimental results fit well with testing data, so the simulation is a practical method in coal measure strata diagenesis. This study can provide references for prediction and exploration of petroleum in reservoir study.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016