Diagenesis in lacustrine organic-rich shales: evolution pathways and implications for reservoir characteristics

Abstract

Both the primary mineral composition and secondary diagenetic processes may affect shale reservoir characteristics such as effective porosity, pore throat distribution, resulting permeability, wettability, etc. Hence, a better understanding of shale diagenesis is key to the prediction of shale oil and gas resource potential. The first member of the Qingshankou Formation (K2qn1) in the northern part of the Songliao Basin is an organic-rich shale with great source potential. Attempting to characterize the influence of different diagenetic processes active in the clay mineral-rich formation on pore space evolution, 19 sample from the K2qn1 interval (vitrinite reflectance from 0.55 to 1.58 %Ro) were selected and investigated by optical and scanning electron microscopy (SEM). This maturity window covers the hydrocarbon generative and expulsion stages and hence allows to reconstruct the processes active during organic matter (OM) transformation. Interactions of inorganic mineral grains with products of the transformation products of lamalginite-dominated primary OM (i.e., soluble bitumen) and associated pore space changes could be observed at various maturity stages. SEM visible authigenic quartz is present from the oil window up to the dry gas window, mostly in the form of submicron (nm-μm) size microcrystals embedded in the interparticle pores between clay minerals. Euhedral and subhedral quartz types are occasionally visible in mineral dissolution pores and OM-hosted pores associated with post-oil solid bitumen. Authigenic clay minerals (such as chlorite) are visible along the whole maturity range, but predominantly form in interparticle and OM-hosted pores at maturity levels >1.1 %Ro. Solid bitumen impregnations are often associated with authigenic minerals, forming rims along crystal boundaries. This indicates that the mineral precipitation may be associated with fluid compositional changes which occur during hydrocarbon generation (e.g., formation of water-soluble organic acids, etc.). According to the SEM observations, clay mineral-associated interparticle pores are the main storage space for bitumen in the K2qn1 source rock reservoir. These pores may be occluded at early to peak oil window maturity and re-opened at post-oil window maturity due to the expulsion of main parts of the generated hydrocarbons (pyrobitumen stage). This highlights that hydrocarbon generation and expulsion are key factors in porosity development both with respect to organic (bitumen generation) and inorganic (e.g., authigenic quartz precipitation) transformation reactions.