Diagenetic and Porosity Evolution of Conglomerate Sandstones in Bayingebi Formation of the Lower Cretaceous, Chagan Sag, Chinae-Mongolia Frontier Area

Abstract

The fan delta conglomerate sandstones of Bayingebi Formation including conglomerates and pebbly sandstones in the Chagan Sag are reservoir for large accumulations of hydrocarbons. Based on observation of cores, thin sections, SEM and CL, combined with application of fluid inclusion and carbon and oxygen isotopes, it indicates that the conglomerate sandstones are mainly feldspathic litharenites with immature texture. It has considerable heterogeneity with a wide range of porosity from 0.3% to 15.3% and the permeability from 0.004 mD to 1168 mD. This study discusses conglomerate sandstones diagenesis and its controls on reservoir quality. Its eodiagenesis is primary compaction and the minor of calcite cementation. The subsequent mesodiagenesis includes weak compaction, precipitation of quartz overgrowth and feldspar overgrowth, albitization, dissolution of early carbonate cement and metastable framework grains, late carbonate cementation, and illitization. A comparison between porosity-depth data and normal compaction curve of conglomerate sandstones shows two groups. Group “A” is mainly composed of tight conglomerate sandstones with permeability less than 1 mD and porosity less than 10%. Petrology studies show that two types of the tight conglomerate sandstones developed in this group. One type, mainly depositing in the thinly bedded subaqueous channels of fan delta front, is characterized by abundant clay minerals occluding intergranular pores. The other type, mainly depositing in the thinly bedded subaqueous channels or along sandstone/mudstone interface of thickly bedded subaqueous channels, is characterized by abundant carbonate cements occluding intergranular and intragranular pores. Group “B” is mainly composed of conglomerate sandstones with higher permeability more than 1 mD and porosity ranging from 5% to 16% corresponding to the burial depth from 2200 to 3400 m. It mainly deposited within the thickly bedded subaqueous channels of fan delta front. Significant dissolution of carbonate cement, feldspars, and other metastable framework grains by organic acids and CO₂ generated from organic maturation are the major cause of abnormally high porosity.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016