鄂尔多斯盆地下二叠统深盆气藏形成的地质条件

GEOLOGIC CONTROLS OF DEEP BASIN GAS ACCUMULATIONS IN THE LOWER PERMIAN SEQUENCE OF THE ORDOS BASIN

深盆气藏在很多方面不同于传统气藏,常具有饱和气体、异常压力、下倾部位无气水界面和低渗透储层等特征,鄂尔多斯盆地下二叠统气藏就是一个典型的直接型深盆气藏的实例。指出:该深盆气的主要赋存层位为山西组和下石盒子组,储层岩石为河流三角洲相砂砾岩和砂岩,砂体厚度大、分布广泛,岩性致密,储层渗透率普遍小于1×10-3μm2(但对于深盆气的储集,仍是十分有效的储层);上古生界煤系气源岩表现为广覆型分布、较高有机质丰度、倾气腐殖型有机质为主的特点,属于全天候式生气,目前虽然可能基本上不再继续供气,但其生成的巨大资源量在极为缓慢的散失速率下仍能形成特大型深盆气藏;深盆气藏主体分布区以常压和低压为主,天然气分布充分表现出深盆气藏特有的气水倒置特征;山西组、下石盒子组深盆气甜点属于地层性质,多出现在深盆气上倾边界附近的储层中。

Deep basin gas accumulations, a type of unconventional gas accumulation, are typically pervasive accumulations. Some of the more important distinguishing characteristics of deep basin gas accumulations include abnormal pressures, low-permeability reservoirs,and a general absence of downdip of water. The Ordos basin is one of the direct types of deep basin gas accumulations. The deltaic deposit constitutes the dominant reservoir in the deep basin. The stratigraphic interval containing the deep basin gas accumulations includes the Shanxi and Xiashihezi formations of the lower Permian sequence. Gas-charged reservoirs in the deep basin gas accumulations are regionally pervaisive, and exhibit low porosity（〈15%）and low,permeability（〈1 md,in-situ）. The deep basin gas in the Ordos basin was sourced from humic,type Ⅲ organic matter contained in coal beds and carbonaceous shales in several coal-bearing intervals. The source rocks associated with reservoir rocks throughout the whole region provide a prerequisite for the unconventional gas accumulation. The gas migration distances in the accumulations are short, perhaps tens of meters or less. The areas within the deep basin gas systems of enhanced reservoir quality （sweet spots） must be identified. These sweet spots in the Ordos basin may be stratigraphic in nature and they most likely occur near the upper boundary of the deep basin gas systems.