Formation of abnormally high porosity/permeability in deltaic sandstones (Oligocene), Bozhong depression, offshore Bohai Bay Basin, China

Abstract

The porosity-enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies. However, the mechanisms responsible for the formation of abnormally high porosity/permeability remain unclear. The Oligocene Dongying Formation in the Bozhong Depression, offshore Bohai Bay Basin, preserves abnormally high porosity, i.e., 36.4% (average) or 42.1% (median) higher than background values. This work selects samples with abnormally high porosity (AHP), samples with normal porosity (NP) and samples with abnormally low porosity (ALP) and compares their differences in physical properties (porosities, permeabilities and thin section porosities), diagenetic minerals, pore structures, compaction and cementation, detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity. An effective export of dissolution by-products (Al3+ and Si4+) from AHP to NP and ALP leads to 2.01% (average) or 2.11% (median) newly created porosity in AHP, which is still significantly lower than the porosity difference between AHP and NP. In comparison, compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling AHP formation. The content and distribution pattern of early carbonate cements, compared to the sedimentary factors (grain size and rigid grains), have significant influences on compaction reduction. Only samples with moderate contents (0.5–3.0%) of uniformly point-distributed early carbonate cements and with effective export of dissolution by-products can form AHP. Three formation models for ALP and two formation models for NP are also proposed. In summary, the evolution of porosity/permeability is controlled by early carbonate cements and induced compaction reduction (major causes) and by feldspar dissolution and redistribution of by-products (minor causes). This work provides new constraints on the formation of AHP and heterogeneity and is therefore important for sandstone reservoir quality prediction.