Reservoir Characteristics and its Main Controlling Factors of the Siegenian Formation of Devonian in X Block, Algeria

The Siegenian Formation reservoir is a tight sandstone reservoir with relatively poor porosity and low permeability. In this paper, the reservoir characteristics and its main controlling factors of the Siegenian Formation in X block have been studied based on cores, well-logs, and formation test data including the thin sections and cast thin sections, observation of scanning electron microscope, pressured-mercury testing and X-ray diffraction analysis of clay minerals. During deposition of the Siegenian Formation, the depositional environment was predominated by lower shoreface, middle shoreface and offshore. The fine sandstone and siltstone formed in the middle shoreface environment is the main reservoir of the Siegenian Formation in X block. Six pore types including primary intergranular pores, intragranular dissolved pores, intergranular dissolved pores, moldic pores, cement dissolved pores and microfractures are identified in the Siegenian Formation reservoir. The quartz intergranular dissolved pores and the quartz overgrowth cement dissolved pores are found for the first time in this area. The secondary dissolution pore is the main pore type in X block. The pore throat has a small radius, poor connectivity and the pore structure parameters are poor. Combined with the petrophysical property and capillary pressure curves, the reservoir of the research area is divided into three types: mesopore-ultra low permeability reservoir (type I), low-middle porosityultra low permeability reservoir (type II) and low porosity-ultra low permeability reservoir (type III). Type II is the main reservoir in this area. The sedimentary environment, tectonism and diagenesis are the main controlling factors. Diagenesis is the most important controlling factor in this area. Diagenesis may play a catalytic role or may play a negative role on damage in the reservoir pore evolution. The compaction, pressure solution, cementation, replacement and cementation are the main diagenesis types which reduce the pore volume during the reservoir pore evolution. The dissolution including acidic dissolution and alkaline dissolution are the main diagenesis types which produce extra pore volume during the reservoir pore evolution. This study of the reservoir will guide the future exploration of this area.