Tight Carbonate Microstructure and its Controls: A Case Study of Lower Jurassic Da'anzhai Member, Central Sichuan Basin

Abstract

Cast thin section observation, scanning electron microscopy (SEM), high‐pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) were used to examine the microstructure of tight carbonate reservoirs in the Lower Jurassic Da'anzhai Member, the central Sichuan Basin. The pore space in the Da'anzhai Member is classified into 2 types and 17 subtypes, with nano‐scale pore throats of ‘O’, ‘S’, ‘Z’, and ‘I’ shapes. Poorly sorted pore throats vary greatly in diameter; thus, it is difficult for fluid flow to pass through these pore throats. There are three classes of pore throats in carbonate reservoirs, i.e. isolated pores, pores coexisting with fractures, and large pores and fractures. Isolated pores may provide some pore space, but the permeability is low. Pores and fractures coexisting in the reservoir may have a great impact on porosity and permeability; they are the major pore space in the reservoir. Large pores and fractures have a great impact on reservoir properties, but they only account for a limited proportion of total pore space. The microstructure of Da'anzhai reservoirs, which dominates fluid mobility, is dependent on sedimentary environment, diagenesis, and tectonic process. Pore structure is related to sedimentary environment. The occurrence of microfractures, which may improve reservoir properties, is dependent on tectonic process. Diageneses are of utmost importance to pore evolution, cementation and growth of minerals have played an important role in destroying reservoir microstructure.

Carbon‐cycle perturbations and intensified continental chemical weathering linked to volcanism during the Jenkyns Event in the Ordos Basin

“…Based on the analysis of the samples from the Fuxian Formation in Well HH105 of the Ordos Basin, some layers contain only a small number of typical Triassic genera, such as Chordasporites , Aratrisporites , Kraeuselisporites , Colpectopollis , Limatulasporites , and Vesicaspora . Representative genera of the Cretaceous, such as Appendicisporites , Cicatricosisporites , Impardecispora , Aequitriadites , and Schizaeoisporites , are also lacking (Li et al, 2021; Liu & Zhang, 1998). The majority of genera and species in the assemblage are common in the Early and Middle Jurassic spore assemblages worldwide, including Cyathidites and Deltoidospora .…”

“…The presence and variation in Triassic relict elements in spore assemblages are important criteria for distinguishing different stages of the Early Jurassic (Li et al, 2021; Liu & Zhang, 1998). Typically, Early Jurassic spore assemblages contain ancient or Triassic relict elements, with their abundance decreasing over time.…”

Identification of fractures in tight-oil reservoirs: a case study of the Da'anzhai member in the central Sichuan Basin, SW China

“…Based on the coexistence of the microfractures and matrix pores, the Da'anzhai reservoir was initially defined as a matrix fracture-pore reservoir. In the matrix state, micropores and microfractures are reservoir spaces, and the connections among microfractures provide the channels for the oil migration 36 .…”

Influence of reservoir lithology on porous flow resistance of gas-bearing tight oil reservoirs and production forecast

“…In tight reservoirs, the size and distribution frequency of pore throats is an important factor affecting flow capacity of reservoirs (Li et al 2020(Li et al , 2017Lin et al 2021Lin et al , 2018. In order to better analyze resistance of gas to cores with different lithology and permeability, microscopic pore throat structure characteristics are first studied using a highpressure mercury porosimeter.…”