A Comparative Study on Microscopic Characteristics of Volcanic Reservoirs in the Carboniferous Kalagang and Haerjiawu Formations in the Santanghu Basin, China

Self-jetting high-yield oil flow was obtained from Ma 67 and Ma 36 wells drilled in the volcanic reservoirs of the Haerjiawu Formation in the Santanghu Basin, China. This has shifted the prospectors’ attention to the Haerjiawu Formation from the Kalagang Formation, which is generally considered to have favorable physical properties. To further explain the geological reasons why oil flow can jet itself from the volcanic rocks in the Haerjiawu Formation with poor physical properties, this study carries out a systematic comparison on the microscopic pore structures of volcanic rocks through unconventional tests such as low-temperature nitrogen adsorption, high-pressure mercury injection, and constant-rate mercury injection based on the analyses of physical properties and minerals. The results obtained are as follows. The volcanic rocks of the Kalagang Formation have relatively high pore permeability. However, their micropores have a wide distribution range of pore size and feature highly meandering structures and strong heterogeneity. Meanwhile, small pore throats connect large pores in the volcanic rocks, resulting in a relatively high pore/throat ratio. All these are conducive to the occurrence of tight oil and gas but unfavorable for the flow of oil and gas. The volcanic rocks in the Haerjiawu Formation have relatively low volcanic permeability. However, small pores connect large pore throats in the volcanic rocks; thus, leading to a relatively low pore/throat ratio. Meanwhile, the volcanic rocks feature low meandering structures, strong homogeneity, and high connectivity. All these are favorable for the formation of tight oil and gas reservoirs. These assessment results also indicate that the assessment indices of tight volcanic reservoirs should not only include porosity and permeability. Instead, more attention should be paid to the microscopic pore structures, and it is necessary to analyze the charging and flow of tight oil from the configuration of pores and pore throats. This study not only explains the geological factors of the wells with self-jetting high-yield oil flow in the Haerjiawu Formation from the perspective of microscopic pore structures but also provides a new idea and comparison method for the assessment of tight reservoirs in other areas.

Reservoir Characteristics and Main Controlling Factors of the Mesozoic Volcanic Rocks in the D Oilfield in Southern Gentle Slope Zone of the Laizhouwan Sag

The Mesozoic volcanic rocks are widely developed in the Bohai Bay basin. The D oilfield, located in the southeast of the Bohai Bay Basin, is a Cenozoic depression developed on the base of the Mesozoic. The types of the volcanic rocks are complex and the reservoir space is diverse. According to the characteristics of the volcanic reservoir, such as vertical multi-stage and strong heterogeneity, and based on the analysis of the volcanic core observation, thin section identification, logging data and seismic data, we analyzed the reservoir space type, physical property characteristics and reservoir physical property control factors of volcanic reservoir in the study area. The results show that the volcanic rocks in the study area are mainly volcanic breccia, andesite and tuff; the lithofacies types mainly include volcanic eruption facies, effusion facies and volcanic sedimentary facies, and the volcanic eruption facies is the most developed. Four types of volcanic reservoirs and 14 effective storage space types have been identified from the macroscopic and microscopic multi-scale, mainly intergranular pores, intergranular dissolution pores, intracrystalline pores, structural fractures and weathering dissolution fractures. Reservoir performance is mainly affected by lithology, lithofacies, tectonic activity and diagenesis. The primary pores in the upper part of exhalative and explosive facies are the most developed. Early cement filling is beneficial to the preservation of primary intergranular pore space and is an important prerequisite for the formation of secondary dissolution pores. Under the action of multi-stage tectonic movement and weathering leaching, the reservoir performance of volcanic rocks has been greatly improved, and the volcanic rocks with superimposed fractures and porosities are effective volcanic reservoirs.