大型咸化湖盆地层岩性油气藏有利条件与勘探方向——以柴达木盆地柴西南古近纪为例

Favorable condition and exploration prospecting of lithologic hydrocarbon reservoir in large-scale saline basin——Case study on the Eogene in the southwest of Qaidam Basin

柴达木盆地古近纪—新近纪经历了多期构造运动、古气候由温湿向干旱演变、湖盆逐渐咸化、沉积中心由西向东迁移的演化过程,地层岩性油气藏的主控因素和发育特征有其特殊规律,以柴达木盆地柴西南富油气凹陷精细勘探区带为重点,通过分析烃源岩展布、沉积演化和主要沉积体系展布,提出咸化湖盆在河流入湖后因密度差较大,易形成密度流等成因的各类滩坝砂体,最大湖泛面之下的高位体系域、湖侵体系域是地层岩性油气藏的主要勘探领域。柴西南古近纪斜坡背景为岩性油气藏提供了基本成藏条件,构造宽缓带和鼻隆带有利于形成地层岩性油气藏,构造背景和砂体成因决定了局部的油气富集,斜坡背景上的宽缓平台区是源下、源内岩性油气藏的主要富集区。地层岩性油气藏的勘探重点是咸化湖盆的各类滩坝砂体、湖侵体系域的滨浅湖滩坝砂体、扇三角洲和三角洲前缘的各类较孤立的砂体,滨浅湖带广泛分布的藻灰岩也是岩性油气藏的重要勘探方向。

In the Eogene and Neogene, Qaidam Basin experienced the multiphase tectonic movement; the paleoclimate evolved from warm-humid to arid; the water in the basin was gradually salinized; the depocenter migrated from west to east. The master factors and developmental characters of lithologic hydrocarbon reservoirs have their own particularity. Taking the fine exploration zones of the southwest hydrocarbon-rich depression in Qaidam as the key point, with the analysis of source-rock distribution, sedimentary evolution and sedimentary system distribution, the author proposes that the density difference increases after the fluvial water enters, then dam and bar sand bodies are formed easily due to density flow. Highstand system tract and transgressive system tract below the maximum flooding surface are the main exploration areas of lithologic reservoir. The Eogene slope settings in southwest Qaidam provide the basic accumulation condition for lithologic reservoirs. The broad structures and the nose-like uplifts are favorable for lithologic reservoirs; structural settings and sand genesis determine the local accumulation of oil and gas; the broad platforms within slope settings are the main oil-gas accumulation areas of downsource and within-source lithologic reservoirs. The dam and bar sand bodies in saline lake basin, the meare dam sand bodies of the transgressive system tract, the insular sand bodies within the fan-delta and deltafront, and the algal limestones widely distributed over the meare are the main exploration domains of lithologic hydrocarbon reservoirs.