渤海海域烃源岩的生气潜力与天然气成因分析

Gas Potential of Source Rocks and Origin of Natural Gases in Bohai Sea

本次研究采用了黄金管的生烃模拟实验分析了渤海海域6个古近系的烃源岩样品的生烃特征并计算了各生烃凹陷的生气强度。研究表明,烃源岩初次裂解气阶段对应的Ro为0.6%~1.3%时,生气高峰对应Ro约为1.2%。Ro达1.3%时,大部分干酪根初次裂解气已生成,对应的产气率平均为122.93 m L/g TOC,达到好的气源岩标准;Ro>1.3%的高演化阶段则主要为油二次裂解气。Ro为1.0%与1.3%时烃源岩的气油比(累积生气量与累积生油量的比值)分别约为0.22(1∶4.5)与0.39(1∶2.6),表明古近系三套烃源岩均具有良好的生气潜力。基于产气率图版计算的各凹陷的生气强度普遍大于20×10~8m^3/km^2,说明研究区具备形成大中型油气田的物质基础。基于已发现天然气的化学组成与碳同位素组成特征,渤海海域现今已发现的天然气主要为有机成因的热成因气,进一步可分为油型气与偏腐殖型气,而且为干酪根初次裂解气,天然气的成熟度对应的Ro值总体低于1.3%,这整体与热模拟实验结论相吻合。与辽东湾地区相比,渤中地区天然气主要为溶解气富集,主要原因是该地区断裂活动强度大,不利于天然气保存。

The gold-tube pyrolysis experiments were employed in this investigate the hydrocarbon generation characteristics of six Paleogene source rock samples in the Bohai Sea area. Based on the experimental results, the hydrocarbon generation capabilities in different sags of the study area were calculated. It is indicated that the main generation stage of primary kerogen-cracking gas of source rock ranges from Ro = 0. 6% to Ro = 1. 3% and the peak of this stage is approximately at Ro = 1. 2%. The majority of the primary kerogen-cracking gas has generated at Ro = 1. 3% ,at an average gas yield of about 122. 93 mL/g. TOC. This reaches the level of good- quality gas-prone source rock. In contrast, secondary oil-cracking gas dominates the gas generation in highly thermal evolution at Ro 〉 1. 3%. The values of G0R（gas to oil ratio,i. e. ,the ratio of cumulative gas yield to cumulative oil yield） ,are 0. 22（1-4. 5） and 0. 39 （1-2. 6） for source rocks at Ro = 1. 0% and Ro 〉 1. 3% Respectively. This indicates good gas potential for source rocks in the Bohai Sea. The values of gas generation capability, calculated by experimental data in this study, are over 20 x l〇 8m3/km2in general. This implies that sags in the Bohai Sea can provide sufficient gas source for the accumulation of medium-giant gas fields. Based on the chemical compositions and carbon isotopes of discovered gases,it is suggested that the gases are mainly thermogenic in origin,which can be further divided to oil-associated and humic-type gases. These gases are derived from the primary cracking of kerogen and the maturity is characterized by Ro 〈1.3%. This is consistent with the pyrolysis experimental resluts of this study. Comapred with theLiaodong Bay,the natural gases in the Bozhong area of this study are mainly oil-dissolved,which may be ascribed to poor preservation conditions due to highly intense fault activities.