Pore structure characterization of the Cretaceous Quantou Formation: Results from microresistivity imaging logs in the second scientific drilling borehole (SK-2 east borehole) Songliao basin, northeast China

As the main borehole of the China Cretaceous Continental Scientific Drilling Project, the SK-2 east borehole was drilled to obtain in-situ multi-information of geophysics and investigate the paleoclimatic and environmental changes. A study based on continuous geophysical logs facilitates achievement of scientific goals, such as the establishment of the typical petrophysical properties for Songliao basin and other similar basins, analysis of sedimentary environment, resource exploration, geothermal system research, and long-term observation and fluid experiments on deep strata. Both conventional logs and microresistivity imaging logs of the Cretaceous Quantou formation have been acquired. Porosity spectra from microresistivity imaging logs are obtained by using the Archie equation. Based on analysis of porosity spectrum features, we divide the porosity spectra of K1q in the SK-2 east borehole into twelve configurations according to the location and combination of peaks to characterize different pore structure. The porosity spectra of K1q in the SK-2 east borehole are dominated by unimodal distribution indicating more primary porosity. Five layers dominated by unimodal distribution are considered for performing long-term observations and fluid experiments on aspects of depth, thickness or petrophysical properties. By comparing spectrum features, porosity spectrum can be used to reflect the heterogeneity and analyze the pore structure of clastic strata. Layered strata will produce complicated porosity spectra as secondary porosity will. It is better to combine porosity spectra with microresistivity image to study secondary porosity. The decline of the porosity heterogeneity in the SK-2 east borehole is demonstrated by two parameters, φvk and W from porosity spectra, which can indicate the sedimentary environment. Moreover, porosity spectra in clastic strata is favorable for determining lithology and sedimentary environment when combined with microresistivity images.