Prediction of Tight Gas Sand Formation and Gas Cumulation in Organic Rich Series
Tight gas sand is one of the important unconventional reservoir types. It is widely distributed in almost all the basins in the world. Its potential is several times larger than that of the conventional reservoir gas. Therefore, clearly understanding the mechanism of tight gas formation will do help to predict its distribution, to enhance the exploration efficiency.
Here, tight gas sand samples were investigated through SEM, microscope mineralogy, and isotopic analyses. Samples were collected from several characteristic tight gas areas including the Ordos Basin, Sichuan Basin, and Tuha Basin, China. The results showed that carbonate is the key to tight sandstone reservoir and its content is inversely proportional with the porosity/permeability. Carbonate is usually concentrated in the tight gas sands with contents above 10%, but below 5% in the normal sandstone reservoirs. In addition, carbonate in the tight gas sand is depleted in 13C, -5~-15‰ (PDB), implying part of C in carbonate origin from organic matter.
Tight gas sands were present close to or interbedded with the organic rich source rocks in all the studied cases. Organic matter must be the inducing factor toward the formation of tight gas sand, because they could generate quantity of CO2 during diagenetic process. Large quantity of CO2 might be generated, far more than the contents in the natural gas reservoirs (CO2 <5%). The lost CO2 might be converted to carbonate in case of the applicable settings with high alkaline formation water (rich in Ca2+/Mg2+).
To model the carbonate converted by CO2 generated from organic matter during the thermal process, we performed sealed gold tube pyrolysis of two low-mature coals. The results showed that ratios of CO2/C1-5 range from 0.5 to 1 at different temperatures (300oC~610oC with two heating ratios of 2 oC/hr and 20 oC/hr). The total carbonate converted by CO2 from 1 m3 coal (Jurassic coal with 67% of TOC), could occupy 0.27 m3 porosity. The two main CO2 generation stages were at the vitrinite reflectance <0.7 % and 1.8~2.1%, respectively. About half of the potential of CO2 is generated from the low mature stage, Ro<0.9%. In addition, the relative generation rates of CO2 and C1-5, as well as isotopic ratios of individual hydrocarbon gases and carbon dioxide, could be modeled. We will present how to identify the time of the tight gas accumulation and sands being tight. Thus the distribution of the tight gas sands can be effectively predicted.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California