Lacustrine marlstones of mixed silisiclastic-carbonate origin are widely developed in the lower third member of the Shahejie Formation(Es

3L

)in the Shulu Sag

Bohai Bay Basin

and are the main target layers for tight oil exploration.Various data including core analysis

thin section observation

porosity and permeability measurement

mercury intrusion porosimetry

scanning electronic microscope(SEM)

CT scanning and nuclear magnetic resonance(NMR) were used to clarify the geologic properties of the special marlstone reservoir of mixed silisiclastic-carbonate origin.Based on the comprehensive analyses in geology and geochemistry

the main controlling factors favorable for reservoirs development were revealed

the criteria for reservoir classification and evaluation were established

and furthermore the favorable plays for tight oil exploration were pointed out.The results indicate that the marlstones in Es

3L

can be divided into two groups

namely lamellar marlstones of interlaminated mixing and blocky marlstones of structural mixing.The mineral compositions of both types are dominated by calcite

followed by dolomite

and are low in contents of clay

quartz

and feldspar.The re-servoir porosity is typically below 4.0%

and matrix permeability less than 10×10

-3

μm

2

manifesting its nature of ultra-low porosity and extra-low permeability reservoir.There are diverse types of reservoir pores with complicated structures

which can be divided into two categories and eight subcategories.Most of the matrix pores are nanopores.Reservoir pro-perties and pore structures are distinctly different between marlstones with diverse structures:the pore distribution in the lamellar marlstones is highly heterogeneous

featuring double or multi-porosity media;the lamellar marlstones are better than blocky marlstones in terms of porosity and pore structure.The favorable marlstone reservoirs of mixed silisiclastic-carbonate origin are controlled by sedimentary subfacies

rock fabric

abundance and maturity of organic matters

and density of structural fractures.As for tight oil exploration

the lamellar marlstones with high TOC that are deposited in inner slope and semi-deep and deep lacustrine subfacies of trough areas are the favorable targets for tight oil exploration.