Evaluation Methods of Profitable Tight Oil Reservoir of Lacustrine Coquina: A Case Study of Da'anzhai Member of Jurassic in the Sichuan Basin

Based on core observation, cast and fluorescent thin sections, FESEM and ESEM, coquina in Da’anzhai Member of Jurassic in Sichuan Basin were examined systematically. Together with production data and logging evaluation, a method for lacustrine coquina evaluation based on geological theory was established up. In the article, two aspects of the study were elaborated, characteristics of favorable reservoirs, and a "five-step" evaluation method for favorable coquina reservoir. According to the correlation between porosity and production data, porosity is not effective in finding high quality coquina reservoir of this area. Whereas micro research of reservoir samples from a high productivity well revealed that sparry coquina is the best lithofacies, with the most developed micro storage space of various kinds. After the favorable reservoir was sorted out, a five-step method evaluating the coquina reservoir was worked out. Correlation of GR value and rock types suggests that GR<30 API is an effective evaluation parameter in identifying profitable reservoir lithofacies. Meanwhile, the combination of profitable reservoir rock thickness and production data revealed that the reservoirs with the highest potentiality are those with thickness of 3–18 m. Fractures are more developed in faults, folds and structural noses in the study area. Organic acid is discharged massively before the peak of hydrocarbon generation, leading to the formation of dissolution pores in the reservoir. The evaluation of organic acid was made by using the source rock indexes. After evaluating the four factors, and compiling their distribution maps, the maps were overlapped to predict favorable reservoir zones, and 7 first class and 9 second class favorable zones of coquina were picked out.