Abstract:

The Lower Cretaceous layer of most sags in the Erlian Basin develops a set of lacustrine dolomitic rocks. The dolomitic layer not only serves as an important stratigraphic correlation marker, but also is an effective hydrocarbon reservoir with a highly heterogeneous complex pore structure. The key of hydrocarbon exploration is finding high quality reservoir. In this study, we took lacustrine dolomitic rocks from the Lower Cretaceous of the Erennaoer Sag as an example to investigate the characteristics and genesis of high quality reservoir of dolomitic rocks. We utilized a multifaceted approach including core observation, petrography, scanning electron microscope (SEM), X-ray diffraction and stable carbon/oxygen isotope geochemistry to better understand the rocks’ petrologic and diagenetic characteristics and their influence on reservoir quality. In order to analyze their reservoir characteristics in-depth, we divided the dolomitic rocks into three types: dolomitic siltstone, dolomitic tuffacecous rocks and dolomitic mudstone. The results are summarized below. Dolomitic siltstone often occurs adjacent to dolomitic/calcareous mudstone. It shows relatively good physical properties with an average porosity of 10.2% and permeability of 1.03 mD, and belongs to low-ultra low porosity and extra-ultra low permeability reservoir. While microcrystalline dolomite can fill intergranular pores or replace matrix or calcite cement, calcite cementation was the major factor for decreasing reservoir porosity and permeability. On the other hand, organic acid dissolution of tuffacecous rich minerals or dolomitic siltstone (adjacent to dolomitic tuffacecous rocks) resulted in intra and intergranular pores. Dolomitic tuffacecous rocks are often interbedded with mudstone and siltstone, occurring mainly in the fore-fan delta subfacies as microcrystalline dolomite or fine-coarse crystalline calcite. They mostly distribute in the tuffaceous matrix as aggregates, with an average porosity of 9.85% and permeability of 0.34 mD, and belong to ultra-low porosity and extra low permeability reservoir. The decreasing reservoir porosity and permeability were primarily attributed to the tuffaceous materials, whose alteration released abundant ionic species into the formation water, resulting in not only early carbonate cementation enhancement but also clay mineral pore filling. In contrast, dissolution contributed the most to reservoir quality improvement as dissolution of carbonate aggregates led to intra and intergranular pores. In dolomitic mudstone, dolomitization is weak and dolomite occurs mainly as micritic crystal along the laminae. The physical properties of dolomitic mudstone are rather poor with an average porosity of 2.5% and permeability of 0.01 mD, resulting in poor reservoir quality. Furthermore, strong carbonate cementation in the late diagenetic stage resulted in the dense reservoir which relies mainly on microfractures for improvements. Although microstructures are prone to develop near the fault zone, their likelihood decreases with increasing distance from the main fault.

Key words: genesis of high-quality reservoir, diagenesis, dolomitic rocks, Cretaceous Arshan Formation, Erennaoer Sag, Erlian Basin