Identification and characteristic analysis of carbonate cap rock: A case study from the Lower-Middle Ordovician Yingshan Formation in Tahe oilfield, Tarim Basin, China

Accurate description of heterogeneous distribution and complex sealing capacity of carbonate cap rock is essential for prospecting carbonate reservoirs. Based on core and thin sections, well drilling, fullbore microscan imaging (FMI) and conventional logs, qualitative and quantitative identifications of cap rock were performed within the Yingshan Formation (O(1-2)y) in Tahe oilfield. Porosity and permeability measurements, scanning electron microscope (SEM) analysis, and displacement pressure calculations were used to characterize the reservoir-cap assemblages. After calibration based on cores and wireline logs, four recognition patterns of cap rock, including blocky-bearing, layered-bearing, porphyritic-bearing and linear-bearing, are developed on FMI. Four sensitive logging parameters (deep-shallow dual lateral resistivity (RD-RS), natural gamma ray (GR), caliper (CAL)) were chosen to construct the quantitative identification criteria, with the higher value being RD-RS >= 1000 Omega m and lower being GR <= 15 API. Four lithologic types and eleven subtypes were identified in carbonate cap rock, and micrite was dominant lithology. The cap rock is characterized by low porosity ranging from 0.04 to 1.4% (averaging 0.568%) and low permeability ranging from 0.004 x 10(-3) to 0.95 x 10 (-3)mu m(2) (averaging 0.07 x 10(-3)mu m(2)). Five sets of cap rock have a significant bottom-up difference in diverse members, and the thickness of individual cap rock ranges from several meters to tens of meters. The stability parameter (S) was adopted to reflect the distribution differences, and superior cap rock defined as the Order I (S > 3.84) occurs in the middle part of the 3rd member. Capillary seals control the sealing capacity, and the displacement pressure difference of effective sealing among the cap rock and underlying oil-bearing intervals should be 3 MPa and higher. Strong vertically superimposed and poor laterally connected direct carbonate cap rock plays a decisive role in controlling hydrocarbon preservation and production of carbonate reservoirs.