Chemical kinetics evaluation and its application of natural gas generation derived from the Yacheng Formation in the deep-water area of the Qiongdongnan Basin,China

The natural gas generation process is simulated by heating source rocks of the Yacheng Formation, including the onshore-offshore mudstone and coal with kerogens of Type II2-III in the Qiongdongnan Basin. The aim is to quantify the natural gas generation from the Yacheng Formation and to evaluate the geological prediction and kinetic parameters using an optimization procedure based on the basin modeling of the shallow-water area. For this, the hydrocarbons produced have been grouped into four classes（C1, C2, C3 and C4-6）. The results show that the onset temperature of methane generation is predicted to occur at 110℃ during the thermal history of sediments since 5.3 Ma by using data extrapolation. The hydrocarbon potential for ethane, propane and heavy gaseous hydrocarbons（C4-6） is found to be almost exhausted at geological temperature of 200℃ when the transformation ratio（TR） is over 0.8, but for which methane is determined to be about 0.5 in the shallow-water area. In contrast, the end temperature of the methane generation in the deep-water area was over 300℃ with a TR over 0.8. It plays an important role in the natural gas exploration of the deep-water basin and other basins in the broad ocean areas of China. Therefore, the natural gas exploration for the deep-water area in the Qiongdongnan Basin shall first aim at the structural traps in the Ledong, Lingshui and Beijiao sags, and in the forward direction of the structure around the sags, and then gradually develop toward the non-structural trap in the deep-water area basin of the broad ocean areas of China.

Kinetics and model of gas generation of source rocks in the deepwater area, Qiongdongnan Basin

In order to investigate the hydrocarbon generation process and gas potentials of source rocks in deepwater area of the Qiongdongnan Basin, kinetic parameters of gas generation （activation energy distribution and frequency factor） of the Yacheng Formation source rocks （coal and neritic mudstones） was determined by thermal simulation experiments in the closed system and the specific KINETICS Software. The results show that the activation energy （Ea） distribution of C1–C5 generation ranges from 50 to 74 kcal/mol with a frequency factor of 2.4×1015 s–1 for the neritic mudstone and the Ea distribution of C1–C5 generation ranges from 49 to 73 kcal/mol with a frequency factor of 8.92×1013 s–1 for the coal. On the basis of these kinetic parameters and combined with the data of sedimentary burial and paleothermal histories, the gas generation model of the Yacheng Formation source rocks closer to geological condition was worked out, indicating its main gas generation stage at Ro （vitrinite reflectance） of 1.25%–2.8%. Meanwhile, the gas generation process of the source rocks of different structural locations （central part, southern slope and south low uplift） in the Lingshui Sag was simulated. Among them, the gas generation of the Yacheng Formation source rocks in the central part and the southern slope of the sag entered the main gas window at 10 and 5 Ma respectively and the peak gas generation in the southern slope occurred at 3 Ma. The very late peak gas generation and the relatively large gas potential indices （GPI：20×10^8–60×10^8 m^3/km^2） would provide favorable conditions for the accumulation of large natural gas reserves in the deepwater area.

琼东南盆地崖城区泄压带流体活动特征及成岩响应

基于琼东南盆地西部崖城地区10余口钻井的实测钻井资料,以及流体包裹体、激光拉曼探针、古压力热动力学模拟、有机地球化学、阴极发光、铸体薄片和碳酸盐胶结物碳氧同位素组成等多项测试分析,剖析了超压顶界面分布以及附近的泄压带流体活动特征,同时揭示了流体活动造成泄压带岩场响应。研究区泄压带可能位于超压顶面附近地层;地层测试、泥浆密度、测井曲线和速度谱资料确定了崖城区现今超压顶面深度主要分布在3 000-4 000 m,而且由构造高部位向低部位加深;各井超压顶面附近的泄压影响范围不同,具体范围可由镜质体反射率Ro得到。现今超压顶面与古超压顶面（泄压流体排放期）深度变化较小。泄压带流体具有相对高温高压、含有酸性和烃类等特征,流体活动使得泄压带成岩场的温压条件和孔隙流体介质发生变化从而影响了水-岩作用;主要表现为：①泄压带地层的Ro和黏土矿物出现提前转变趋势;②有机质Tmax异常小和S1/（S1＋S2）异常大;③泄压带储层发生的热流体酸性溶蚀导致次生孔隙带发育;④长石颗粒钠长石化,自生石英和碳酸盐胶结物异常发育,其为深部超压有机流体排放的产物。总之,研究区超压顶面附近的泄压带可能为天然气与优质储层及盖层耦合有利聚集带,是今后琼东南盆地天然气勘探的现实区域。

琼东南盆地北部中生代凹陷特征及油气成藏条件初探

琼东南盆地的中生代凹陷群是扩大油气勘探发现的远景区。为了实现琼东南盆地中生界的勘探突破,急需落实中生代凹陷的油气成藏条件和有利勘探领域。基于地球物理、地球化学、钻井资料及古生物资料,首次对琼东南盆地中生代凹陷开展了深入系统的分析研究。研究表明,在琼东南盆地北部崖北凹陷、崖城凸起、崖南低凸起和西南部陵南低凸起及南部隆起均存在中生代沉积地层。根据盆地构造演化分析,中生代沉积形成于燕山期太平洋板块向欧亚板块俯冲期以及其后的弧后伸展期,存在烃源岩、储层及圈闭等油气地质基本条件。琼东南盆地北部坳陷带崖城-崖北地区相比盆地其他地区中生代沉积凹陷规模大,因此本研究重点对琼东南盆地北部坳陷带崖城-崖北地区进行了系统的油气地质综合分析研究,落实了崖城-崖北地区中生界沉积地层分布,并初步评价预测了有利油气勘探区带及重点目标。研究表明,琼东南盆地崖城-崖北地区中生代凹陷发育湖相烃源岩、三角洲等储集体以及构造与地层圈闭,存在西洼南坡带和中部隆起带2个有利油气勘探区带。在这些区带加大勘探,有望获得中生界油气突破。

琼东南盆地崖城地区储层古孔隙面貌恢复

关键油气成藏时期的古孔隙面貌对油气成藏至关重要,前人有关琼东南盆地碎屑岩储层特征与埋藏—成岩演化过程—储层物性演变规律—关键油气成藏时期的时空匹配关系以及相关的系统性研究甚少。基于深层优质碎屑岩储层形成过程的全生命周期理论,通过粒度分析、岩石薄片、铸体薄片、X射线衍射、扫描电镜、压汞、物性分析和碳氧同位素分析等多种分析手段,对琼东南盆地崖城地区陵水组、三亚组和梅山组砂岩储层岩石学特征、物性特征、孔隙类型、影响储层物性的宏观作用(物源、海平面变化、坡折带类型、超压)与微观作用(成岩作用)进行了系统分析,并划分了成岩演化阶段与成岩序列;在此基础上,采用成岩序列法,以孔隙度与时间、深度关系为切入点,将孔隙度增减过程与沉积物形成的“源—渠—汇”过程、成岩演化史、埋藏史三者结合,建立了孔隙度随深度和时间演化的关系,明确了崖城地区关键油气成藏时期储层古孔隙面貌。陵水组古孔隙度分布于2.3%~24.1%之间,古渗透率主要分布于(10~600)×10^(-3)μm^(2)之间;三亚组古孔隙度分布于2.6%~28.4%之间,古渗透率主要分布于(100~1 600)×10^(-3)μm^(2)之间;梅山组古孔隙度分布于7.58%~32.62%之间,古渗透率主要分布于(10~500)×10^(-3)μm^(2)之间。同时定量表征了孔隙演化对储层成岩作用强度的响应,以期为该本区中深层油气勘探提供指导。