The Studying Of Sulfur Contented Oil And Gas Formation From Well Niudong Buried Hill Tectonic Belt In Baxian Depression

Well Niudong1is a risk exploration well which is located at Niudong buried hilltectonic belt of the Baxian Depression, Jizhong Sag. The bottom temperature of thewell is201℃. After the reservoir acid fracturing treatment, it produces560,000cubicmeter gas per day. With the oil and gas development and exploration, trace amount ofhydrogen sulfide (H2S) was noticed. In order to understand the origin of the sour gasand mechanism of oil thermal cracking in the Niudong1area, the source rock andcrude oil pyrolysis experiments were performed in the present study.Thermal simulation experiments using the gold tube were performed on varioustypes of samples including source rock, crude oil, fracturing fluid and drilling fluid.All fluid samples were collected from Well Niudong1, while the source rock wascollected from the fourth member of the Shahejie Formation (Es4) at Well Jing53.Heating rates of2℃/h and20℃/h were applied in gold tube pyrolysis system.Quantity of hydrocarbon gas and sulfur isotope of H2S which generated during theexperiment were analyzed. Based on geologic condition and sulfur isotopic value ofH2S in the produced gas, the origin of H2S was proposed.Our experimental results show that crude oil is not directly cracked into gas, butinto some short chain molecular intermediates (light hydrocarbon, condensate oil)before further cracking. Current Niudong1condensate is not directly derived fromsource rock at high maturity stage, but is formed from the thermal cracking ofprevious existed oil. Condensate oil in the Niudong area has high gas generationpotential during further cracking with up to800ml gas per gram oil. However,because the condensate is mainly composed by saturated hydrocarbons (>95%), thehigh activation energy results in the condensate very hard to cracking. In geologicconditions, large quantity of oil cracking occurs at temperature above230℃. Theoccurrence of H2S in the Niudong area is formed by thermochemical sulfate reduction(TSR) between residual oil and gypsum in the reservoir. Increasing H2S content withdevelopment time is probably caused by further reaction between HCl from fracturingfluid and pyrite in the reservoir.