Sedimentary and diagenetic control on the porosity and permeability in Lower-Middle Jurassic reservoirs, northern margin of Tianshan, China

The Lower-Middle Jurassic sandstone reservoirs in the northern margin of Tianshan are considered a significant area of tight sandstone gas. Sedimentary, petrologic and petroleum geologic criteria have been surveyed in six sections along continuous outcrops and cores. Thin sections and scanning electron microscope (SEM) images combined with mercury injection techniques are used to restore the evolution of pore-throat structures. The Lower-Middle Jurassic sandstones are mainly low-porosity and extra-low-permeability reservoirs. The results of constant-rate mercury injection reveal that the distribution pattern of the pore radii is not related to permeability, whereas the throat radii increase with increasing permeability. The microcosmic heterogeneity in reservoirs is dominated by a high proportion of narrow throats. An analysis of the reservoirs' physical properties and diagenesis indicates that the quality of the Lower-Middle Jurassic sandstone reservoirs is controlled by sedimentary environment, eodiagenesis, and diagenesis that developed in different sedimentary microfacies. The pore-throat structures can be divided into six types. In coal-bearing formations that are rich in organic matter, compaction decreases the quality of the reservoirs, and the reaction of early-formed kaolinite with remnant, undissolved K-feldspar may cause diagenetic illite to block the throats. At the bottom of the fluvial channels in braided river delta plains or at the top of mouth bars in braided river delta fronts, eodiagenesis can inhibit compaction via early basal carbonate cementation, and the unfavorable reservoirs are extra-low-porosity and extra-low-permeability sandstones. Infiltration-related clay is widespread. Smectite coats covered grains, and kaolinite filled remnant intergranular pores as aggregations, which increased the heterogeneity of the pore-throat structures. Authigenic chlorite coats protect intergranular pores from compaction and cementation by quartz overgrowths. Reservoirs with medium to low porosity and medium to low permeability come from the middle-upper portions of fluvial channels in braided river delta plains of the Xishanyao Formation or from the upper part of sub-water fluvial channels in the Badaowan Formation and the Sangonghe Formation or mouth bars in braided river delta fronts of the Sangonghe Formation. In near-surface settings, such as the bottom of fluvial channels or the top of mouth bars in braided river deltas, infiltration-related dissolution leads to more intergranular dissolution pores. However, the development of the highest-quality reservoirs, i.e., medium porosity and medium permeability, is limited. Other types exist between these end members in quality. Due to good sorting and low matrix content, the barrier bars of the Toutunhe Formation and the Sangonghe Formation with good lateral connectivity in the lacustrine facies are widespread in the northern margin of Tianshan and are considered to be high-quality reservoirs.