Saudi Arabia’s deal to buy American natural gas is part of a decadelong $160 billion plan to build up its gas assets, as the oil-rich kingdom’s demand for new energy is projected to soar beyond its capacity. Saudi Arabia foresees a natural-gas empire that fuels futuristic new cities and helps develop local industries in manufacturing, mining and technology. The kingdom produces enough crude oil now to meet electricity demands, but Saudi leaders are trying to stop burning petroleum to create power because it lowers its biggest…